Earlier quoted context omitted.
It's not about the suit, it's about the message. As an employer you want to hire people who make an effort, and want to avoid people who can't be bothered to. Dressing well for an interview is so obvious it's actually a pretty good way of sorting candidates. This may apply less for programmers and high-end technical positions, but this is the exception not the rule.
What you're overlooking is that attention is a limited resource. Someone who pays attention to how they dress has thereby paid less attention than they might have to something substantive. So not only don't we care how people dress at YC interviews, dressing up is actually a (minor) red flag. We'd rather you spent that effort on something else.
Some people use clothing as a means of mental preparation for entering a non-casual situation. I agree there's some empty ceremony to wearing a business suit, but I don't think that's all there is to it.