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.
This is actually already happening, and it has played out horribly in real life. This is what has caused our north american urban sprawl and has demolished most of our downtown areas. It hurts small businesses because they can’t just open a cafe in a small town, now they also need to build parking next to it. It spreads everything out and makes car dependence worse. On top of that, there is no consensus on how much parking is “adequate”.
This video is fascinating, and changed my perception of parking: https://www.youtube.com/watch?v=OUNXFHpUhu8