>>1923470They think of cycling as a hobby, not a way to get places.
So they think that their road space is being blocked by somebody who wouldn't otherwise be there. So they think that they're one more body on the road getting in front of their car and slowing them down and adding to gridlock - and if you weren't on your bike on the roads you'd be at home and out of the way.
What they don't realize is that you'd be on the road either way, you'd just be another asshole in one more car on the road, making them take longer to get places and adding to the gridlock.