It more or less boils down to having little constructive to do at endgame.

For some it’s about a look they really enjoy and want to get right whether it be color scheme or cosplay or whatever.

For some, they really really think other people are jealous or have some other need to show off to other players...think of it like buying an obnoxiously loud and expensive sports car.

Others still just enjoy collecting.

It very much mirrors real world fashion.