I mean, even "monsters" that replicate normal people, like the Gladiator, get things like "Brute: add an additional die of damage to weapon attacks". Most enemies, I honestly can't tell why they do a particular amount of damage at all, it sometimes seems like a truly random and arbitrary amount of dice. Going back to the Giant Ape, we see it's attacks are:
Multiattack: The ape makes two fist attacks.
Fist: Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage.
Rock: Ranged Weapon Attack: +9 to hit, range 50/100 ft., one target. Hit: 30 (7d6 + 6) bludgeoning damage.
Ok so, it gets 3 dice of fist damage because it's huge, but why does it's fist do a d10? Reasons?
And how did one decide a rock it picks up does 7d6 damage? I mean, it's not like it even says how big said rock has to be, or if it has to take an action to pick it up or anything! Did they go "well, it's 15 HD so a level 15 Rogue has 7d6 Sneak Attack"? Or was it "well it's CR 7, so uh, 7d6?" Who knows!
And then when one of these monsters throws a crit, it's going to be way more bonus damage than a level 7 martial character is going to get, which seems fairly disproportionate. Yeah level 7 Fighter crits for another d12 with a greataxe for an extra 6.5 damage. Level 7 Ape crits for another 3d10 (16.5) or 7d6 (24.5) damage.
To compare the two, a player crit is pretty much weaksauce, save for Paladins and Rogues and, I don't know, Half Orc Barbarians at higher levels?
Or the occasional attack roll spell, which is one of those "when the stars align just so" moments.