It impedes traffic. Plain and simple. Roads are built to be access-ways, not a place to store your private property.
Would you object to be storing a pallet of shingles along the side of the road? It takes up less space than your car!
It’s even worse if you park in front of someone else’s house, as it blocks a small bit of access/use of that property too. Now I can’t put my bins out on the curb where I would like, or you parked in from of them so they were not collected.
…and it’s even worse still when there is a long chain of cars parked on both sides of the street. It is plain to see that the road no longer has the same utility. It used to be bidirectional, and now it’s a single lane, and the responsibility is diffuse as no one car owner feels responsible for the hindrance (it’s all the OTHER cars).
It’s not my fault that you have too many cars or not enough driveway/garage space on your property. Keep it out of the public right-of-way.
I agree with this for long-term parking, but I’m not upset by occasional short term parking in the street. I can’t really expect my neighbor’s holiday guests to squeeze 4 cars into a one car driveway.
It also depends on the location I think. My street is extremely wide, so it’s not a big deal. On narrow or crowded streets I would probably feel differently.