People who dress "nicely" to travel through airports, why?

I get that some people have business waiting at the pickup and wanting to look professional, but why everybody else?

Human bias. People tend to treat you favorably when you dress a bit nicely. You don’t have to over-do it, but you also don’t want to appear “rough”. I’ve travelled both ways, and can tell you for a fact, you get hassled more if you look like you just rolled out of bed, as opposed to someone who’s at least trying to look presentable.

You nailed it. When I wear a T-shirt and have my tattoos visible, security treat me like shit and I always get pulled aside for extra checks and drug swipes. If I wear a nice suit, I get smiles and no hassle.