I think I have a rough idea of what's going on. Obviously this doesn't cover all of the player base that likes to drive fast, but this should give you somewhat of an idea.
Some of them do it for content, they make these "dangerous manoeuvres" videos and it shows them doing these dangerous overtakes. Sometimes on the grass, while driving very fast, and usually down the famed C-D road (which I condemn, of course).
Others don't have a care in the world, because it's just a game and they can drive as fast as they want. I would usually tell these sorts of players to drive on Arcade server if that's the case.
Some of them think the designated speed limits set by SCS are too slow.
You'll come across some good drivers who obey the traffic laws and speed limits every now and again, and it's great when that happens. Personally I like to drive around 90km/h.