Because they take up the whole trail and don't know how to drive!
DUH!
We have quads out there that try to go on single track trails where there are sings that clearly say "ATV's not permitted" there's a reason for that sign!
I got t-boned by a quad runner. I was sitting along the side of the pits ( i got off already ) and was watching the quads go around and some kid lost control and hit me. He was going about 30-35mph
Haha yeah, I've had that almost happen to me before
Its scary as shit, luckily my bike was still on and I got outta the way. There was another time where I almost hit on head on ona trail.