Why do asexuals people wear sexy clothes or makeup?
Many asexual people find it inherently rewarding to look and feel good / beautiful. They aren't mistaken about what they really want, or trying to be a tease, or anything else. Some people might try to look beautiful because they want to sleep with people, but it's not as if that's the only reason to do so. It feels bad to dress poorly, and that's because both 1) you see yourself and you don't like how it looks; 2) you know that other people can see you. Humans are social creatures and it's very normal for a person to be in tune with how they are seen by others.