Firstly space is big...
And satellites tend to all be going in roughly the same direction (you get a boost from the rotation rate of the earth if you launch west-east).

Low earth orbit (Shuttle+ISS) is the busiest, with lots of spent booster rockets and most of th emanuouverable satelites. But the drag from the upper reaches of the atmosphere will soon de-orbit any small pieces of junk like the residue of a collision or a dropped toolkit ;-)

Mid orbit (GPS, Iridium) is the worst, there are lots of satellites all going in different full earth coverage orbits and often not steerable. So far it's been lucky. It was a little better when the military (USA+USSR) where the main users since they tended to have a more robust attitude to safety and disposing of old satellites, now there are a lot more commercial lunches in this area it's getting bad. It's hard to convince shareholders to pay another few $10M and delay their service for extra safety systems or testing.

Geostationary (communications and TV) is going to get bad. Although there is a lot more space (in space) this far out there are only a few places you want to put satelites, over USA, Europe, Japan+Korea etc. They don't normally move but have small positioning thrusters to stop them drifting into each other. So far they have all been successfully put in parking orbits at the end of their life but an explosion in one of these congested spots would be a real pain. Any debris is going to stay there pretty much for ever.