The problem with online people is that most of them are flat out liers. Not just the body - everything. They lie about being outgoing, or ambitious, or athletic, or whatever. The shortcomings in looks are just another part of it. Every gay guy describes himself online as athletic and butch...
I know it doesn't make any sense, but people with a little tan look healthier. For me, yeah, it's about vanity. It hides some of the imperfections and boosts my confidence.