It's an easy thing for a third party to check, easier than evaluating technical skill, and easy for a motivated person to fulfill. As such, it also serves as an easy filter that has a lot of false negatives, but probably fewer false positives than a non-subject matter expert trying to evaluate expertise.

It also signals consideration for other human beings, not expecting them to jump through a bunch of expertise evaluating hoops, but instead allowing them to rely a commonly agreed upon and decently useful signal. Consideration for others is yet one more important thing to signal.

It's circular to call that considerate. You're saying it's useful because people care, I'm saying people care about something useless.

You can apply the same logic to an obese person as chronically under-dressed/fashion-averse one: They ought to be considerate enough to show their attention to detail through managing their health better and careful calorie intake.

I stand by my point that it's a shortcoming of our culture that we give those signals (i.e. "t-shirts aren't very professional!") any weight at all.

I won't claim an inverse trend but I've worked with enough relatively-useless and very well dressed people that it's basically noise.

Judging appearances is useful as a very low level check, so I wouldn't say it is completely useless, though I agree it is over emphasized.