Useful low orbital shells around the Earth are an awful lot smaller. They're still big, like the sky is big, but airplanes do manage to, at least somewhat regularly, collide with each other "in the middle of nowhere" (most midairs are around airports, since there are a lot of airplanes heading in and out of them).
Aircraft debris, however, falls out of the sky in short order. Orbital debris doesn't.
The probability of an initial collision is fairly low, but once you have a few, or someone decides to show off their new ASAT weaponry (which is functionally a satellite collision, just one of them only having been in space for a short while), you now have flak that won't go away, moving in all directions at orbital velocity. That, then, has the potential to start causing all sorts of problems, and once that's taken out a few more satellites with orbital-energy impacts, you start having a real problem.
Will it happen? Those invested in littering certain useful orbital shells with satellites claim not. We'll see.