There's atmosphere up there, so it creates drag which slows the stuff down and causes it to re-enter and burn up or crash onto the planet in only a few years. C

There's atmosphere up there, so it creates drag which slows the stuff down and causes it to re-enter and burn up or crash onto the planet in only a few years. Compare that to higher orbit, where there's much less drag and space junk could last centuries or more before it deorbits.