Because unless you're really damn good and don't have any regard for consequence, whatever you hit will probably look kind of lame. It's very time consuming, it's expensive to get speed and lights, and organizing a crew in high school is hard.
I'm all for urban, love watching it and I've had a lot of fun doing it. It's just a lot of work for little reward personally. If everywhere was a winter wonderland with unlimited build capability and you could just pound the zero key to get speed like you're playing shred sauce in real life then yes. Skiing would be like skateboarding where street is the only thing that matters. But since that's not a thing, I'll just keep playing urban maps on shredsauce...