It is indeed a case of divergent evolution, though not one most people would expect. While both sawk and throh fill the same niches, occupy the same territories, and generally live extremely similar lives, the only difference between them is, quite literally, their fighting styles. Fossil evidence shows that they had once been the same species millions of years ago, but somewhere along the lines, the species split into two groups: one that practiced a technique that centered around rapid, hard-hitting precision strikes and the other preferring a style centered around defenses and overall body strength.

It is perhaps the pettiest example of divergent evolution scientists have ever discovered among pokémon.

Please don’t court humanoid pokémon. There are quite literally laws against that.

In all seriousness, though, sawk and throh wear robes because it’s part of their training to make them. While, yes, sparring and punching and kicking things are part of their training as well, there’s a certain level of patience and skill required for sewing and making clothing. A sawk or throh that has put together a full outfit is said to have completed much of its basic training and is, thus, an advanced student of their respective branches of martial arts. And yes, if you took those robes off, they would feel naked, but this is largely in a metaphorical sense, rather than a literal one. Because a sawk and throh’s robes are symbols of how far they’ve come along in their training, to be without them would basically to be stripping away all evidence of how strong they are. Thus, most sawk and throh feel vulnerable when literally naked.

Meanwhile, for the most part, other humanoid pokémon don’t actually wear clothes (they’re simply skin flaps or otherwise parts of their bodies—such as jynx’s “dress,” for example), but those that do simply wear clothes because that’s part of their societies, much like how humans wear clothes because it’s just part of ours. Of the humanoid pokémon that don’t actually wear clothes, many of their genitals are simply either covered by flaps of skin or plates (such as in the aforementioned case of jynx), or they possess internal, rather than external, sex organs.