Why are casters OP in D&D 5e?

TLDR: Most tables ignore the complexity around spellcasting mechanics.

Lots of tables do this:

1. Ignore Material costs to a spell
2. Ignore the somatic and material component requirements to the spell.
3. Don't follow the adventuring day mechanics (basically, they are allowed to rest too often).

The gist of the material and somatic requirements... Unless you have the Warcaster feat, you need at least one free hand to cast a spell (and in some situations where a spell has both material and somatic components and you're casting it with a focus, you don't need the free hand). The problem is people want to have a weapon and a shield -- and the the ability to still cast spells.. instead of the traditional staff (spell focus) in one hand and a free hand to cast/channel the spell.

If you spend some time looking at the material and somatic components to a spell, you'll find that most of the offensive spells have somatic components but not any material components until higher levels (unless you look at Evocation spells which seems intentional).

The Cleric spell list is one of the most important spell lists to look at because it takes the spellcasting focus problem into consideration: You're basically trading the offensive spell capabilities by using a weapon. Virtually all of the support spells require material components -- Bless, Protection from Evil and Good, etc. -- but the attack spells -- Toll the Dead, Guiding Bolt, Spiritual Weapon -- require somatic components only. This means a cleric should always have their hand free unless they are intentionally engaging in melee/martial combat.

Some Design Problems

5e is incredibly lenient on action economy. If a weapon is already drawn, you can drop it (not an action), cast a spell, and then pick it up as part of your move action. Technically speaking, you may not be allowed to do multiple "free actions" in a turn but you'll get into inconsistent arguments about what you can or can't do using free actions. Could you run to a door, release the grip on your two-handed weapon, open a door, then re-grip the two-handed weapon to make an attack? Most DMs would say yes but you've interacted with your two-handed weapon twice and opened a door.

Picking up a weapon doesn't take an action.

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/6288-is-dropping-an-item-actionless

https://www.reddit.com/r/DMAcademy/comments/ua215b/dropping_weapons_as_a_spellcaster/

https://www.reddit.com/r/DMAcademy/comments/ua215b/comment/i61h518/?utm_source=share&utm_medium=web2x&context=3

    I don't want someone like a Paladin or a Cleric to be able to easily cheese their way out of not having War Caster.

My point, overall, with this comment is that I can't imagine any of this as "cheesing". It's strange to me to worry about that, because the game itself makes War Caster very poor in value for that bullet point. The other two features it provides (advantage on Concentration checks, and spells as opportunity attacks) are far more valuable and relevant.

Paladins & Clerics were created to be casting most of their spells with hands full without needing War Caster, and any spells that they do need a free hand to cast are ones you are unlikely to need in combat, or will know to prep before combat since they last long enough for that to be reasonable, or they have alternatives that aren't spells for ways to deal with the problem (in the case of Paladins).

I get this understanding from the Holy Symbol, from the Equipment section (not the Class section, or the Spellcasting section) of the PHB:

    A cleric or paladin can use a holy symbol as a spellcasting focus, as described in chapter 10. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield.

Holy Symbols are special, in that they don't have to be held in a hand to be a foci. They just have to be displayed visibly. Why this is in the Equipment section and not Class/Spellcasting, IDK.

This means that, RAW, a Paladin or Cleric can cast the following spells without War Caster with a Weapon in one hand and a Shield in another, while wearing their Symbol visibly (it doesn't have to be on their Shield):

    V

    M (though there are no M only spells)

    V, M (the only one it really helps out with to just have it displayed)

And they can cast these as long as the symbol is on the Shield without War Caster:

    S, M

    V, S, M

Because the Somatic can be supplied with the same hand holding the Material, and the Material is the Shield, which is bearing the Holy Symbol.

Which means the only spells War Caster helps out with for them are:

    V, S

Which are 19 of 48 Paladin Spells or 39%.

Of which, 9 are unlikely to be ones you'd cast in combat. Examples of these are:

    Death Ward (it's 8 hours, why would you not cast this before combat)

    Find Steed/Greater Steed (10 minute casting time; you can't cast this during combat)

    Create Food and Water/Purify Food and Water

    Zone of Truth

    Detect Evil and Good (They have Divine Sense for combat situations) and Detect Magic (usually not needed in combat situations).

    Remove Curse (in 99% of cases, Remove Curse is fine to put off casting until after combat)

The other 9 are:

    Cure Wounds (but Paladins have Lay on Hands, which, despite the name, doesn't require an empty hand, and only ever says "touch a creature", and not "touch a creature with a free hand".)

    Divine Favor (there are usually better actions to take, unless constant radiant damage is required, such as against a Vampire to prevent regeneration; otherwise, Smites suffice)

    Heroism (there are usually better actions to take, unless frightened is a problem)

    Lesser Restoration (but they have Lay on Hands anyway for most of the effects of this spell)

    Protection from Poison (needed when poison is involved, but lasts an hour so likely to be cast outside of battle any time it's known before hand; again, Lay on Hands)

    Daylight (only necessary when visibility is an issue, so rarely)

    Dispel Magic (rarely a useful action over just killing your enemy and dealing with the spell effect later)

    Magic Weapon/Elemental Weapon/Holy Weapon (but these are an hour, so a lot of the time, it'll be cast before combat begins, if the combat is known to be coming soon)

In other words, every single one of them is highly situational, or can be cast ahead of time when the combat is known to be coming soon.

    38 of 117 Cleric Spells or 32%

I'm not going to go through these like I did the Paladin spells, but they share a lot of the same spells, and since the Cleric spell list is larger, alternative spells to supply the same effect are available. i.e. Healing Word provides healing as Cure Wounds does but isn't a Vocal & Somatic spell.

... but this isn't limited to Paladins & Clerics.

Sorcerers, Wizards, Warlocks, Druids, and Rangers all have an Arcane or Divine Focus via a Staff, which is also a Quarterstaff, which is a weapon and a foci in one object.

This means they functionally serve the same purpose as a Shield with a Holy Symbol on it does for Paladins & Clerics.

The only class that completely lacks a weapon-foci combo is the Bard, but, short of Valor or Swords, they usually don't need to hold a weapon and a Foci at the same time. Swords Bards can use their weapon as a foci though... so only Valor is left as the only Bard that has to deal with free hands and foci for spells.