Actually, I think it's the aliens.

They don't want to waste the effort to deal with us going off-planet, so they've induced us to put so much crap in orbit that eventually it will be essentially impenetrable to human spaceflight, thereby containing our virulent species and protecting the rest of the universe.

Castigat Ridendo Mores
(laughter succeeds where lecturing fails)

"Those who will risk nothing, risk everything"