It's not so much that they're bad, but that they aren't good
The main thing you get out of buying and using an RPG somebody wrote, aside from incidental setting "fluff", is rules, especially for conflict resolution and world generation. But rules in storygames invariably boil down to "make shit up", just like little girls holding a pretend magical tea party, thus the epithet "MTP".
Now, there's nothing wrong with using MTP sometimes, there's even a discreet reminder about it in D&D with "rule 0". The problem is, every child already knows MTP, so anything written inside a published tabletop RPG must be different from and superior to MTP for at least some tasks, otherwise it's just a book full of "fluff" instead of a functional RPG.
Aside from weak or outright nonfunctional rules, however, actual rules and guidelines typical of storygames are also corrosive. For instance, fate points and narrative control disassociate players from their characters, and discourage detailed worldbuilding, in favor of vague worlds that leave room for limitlessly escalating bullshit:
>the wolves press you to the cliff's edge
<i jump off
<so you fall to your doom?
>*burn a fate point* *POOF* *a branch appears*
>no, i grab a branch and climb back up after they leave