The air traffic is a lot more guided. Airplanes don't go anywhere unless approved. Satellites are just circling in different directions and constantly crossing paths.

Luckily most sats in LEO do have maneuvering thrusters. But the fuel is very limited and the more of them are up there the more often they have to do corrections. Eventually something will have to give.

All satellite orbits must be pre approved before a spacecraft can ever leave the ground. They're more heavily regulated than aircraft.

Every satellite is tracked. Potential collisions are monitored and avoidance maneuvers performed. A significant amount of air traffic is completely uncontrolled and relies on looking out the window to avoid collisions. Satellite paths are a lot more predictable, and there's also way more volume to work with.