A couple reasons. One, the stunts are all example stunts. Sure, players can take them (and should, if they precisely fit their concept), but they're only there to give concrete instances of the rules guide they started with.
Two, everywhere that they can, they try to make sure that all the examples are representative of approximately what the RPG designers would want in their games. Not what should be in every game, but 'when we roll, this is how we roll'. They're often selected because their particular flavor is exemplar of the relative flavor our authors prefer... but not because they're what the authors expect us to run.
They've said it time and time again, it's our game once we're holding it.. My attempts at alterations have mainly been to verify "Are you sure this is the tone you want to convey?"
So if you have concerns about the Martial Arts tree, approach it analytically, make comparisons, make suggestions for the apparent tone, and ask them yourself if this is what they actually wanted.
Then you'll have your answer. Me, I'm too busy giggling over enchanted items, but that's probably the drugs talking.
("Oh, I get it, it's a MAAAAAAGICAL diorama." "Shut up, it's a precise arcane instrument." "That looks like you should populate it with lego men." "SHUT UP.")