It’s just people having fun on bikes man. You get a few people who are actually couriers, a few people who are massive try-hards and everyone in the middle who just likes riding bikes with one gear and no freewheel. Some people ride with brakes, some without. Some wear lycra, some wear tight jeans and vests.
If you look at LFGSS, a lot of people who got into fixed gear in 2007 cos it was cool are now involved with road racing, CX, track racing, mtb and TT’ing. I started riding fixed gear cos it was ‘cool’, now I’m a member of a cycle club and race TTs and hillclimbs fixed. More people on bikes = good thing.
Regarding how people dress, it’s just group behaviour innit? People mimic the behaviour of others in their perceived group, which is why you wear baggies to MTB in rather than the far more practical lycra.