I think it's more than just people being careless. Spaces are too small in a lot of car parks. Cars are bigger than they used to be, even the wee ones. If you parked (properly) two average cars like a Focus and an Astra together in a lot of car parks then there's still not going to be loads of room to open the doors.
People are careless or just don't give a damn too, that's fairly obvious.
Thirdly you've got people who just can't park. They're the source of lots of problems - either dings to cars they hit, or because they've parked so poorly other people have to adjust how they park next to them and the knock on effect leaves cars too close together.
Whilst taking two spaces in an empty retail park car park when there's 500 spaces going free might be alright, it's certainly not acceptable in busy places. There's an ####hole that takes 2 spaces at the station. There's not enough spaces to go around, and people usually invent spaces as it is. I think he learned his lesson when someone actually parked beside him anyway.