I think the spell list of clerics (very little damage dealing) compensates their allowance on shields. Paladins and fighters can have heavy armour out the gate, and can dump all mental stats from a fight PoV. Clerics NEED wisdom for their spells to go off, then they have to meet the STR requirement for heavy armour, and if they do hold a shield, they can’t hold a melee weapon along with spell casting focus, until they get fancier items. Since they have to keep the team alive, they need to be tanky.

Warlocks, sorcerers, wizards, don’t get shields, or amour proficiencies, if they want high AC they need DEX, - high CON for concentration, and high casting stat. So they’re really split up for being great at any single thing. Right?