orbit - Why doesn't all space debris burn up?

The NASA Orbital Debris web-page gives some interesting statistics :

12). How long will orbital debris remain in Earth orbit?
The higher the altitude, the longer the orbital debris will typically remain in Earth orbit. Debris left in orbits below 600 km normally fall back to Earth within several years. At altitudes of 800 km, the time for orbital decay is often measured in decades. Above 1,000 km, orbital debris will normally continue circling the Earth for a century or more.

So If we stopped sending things in orbit then eventually it would clear up. But we are probably sending more things up than are coming down.