Satellites fail (lost control, or spontaneously explode). Very small debris is everywhere and under 5-10 cm largely untracked, but some are working to fix this gap.

There is no incentive large enough for cleanup (it's expensive, nobody can/wants to pay, and there are a lot of objects)

Leo starts at 7.8 kilometers per second and speed plus secondary and subsequent collisions with very small debris makes it impractical. Also carrying fuel to react frequently would dramatically change the entire mission.

If your non refuelable sat is good for 6 months it probably no longer makes sense to launch it.

With currently deployed tech, most of the smaller orbital debris is not (usefully) track-able.

At orbital velocities, you gotta know it's coming to be able to avoid it.

And a orbital velocities, the untrack-able stuff can still kill a satellite.