The simple answer is that we don't have a telescope powerful enough. This strikes some people as counterintuitive; we can see stars and nebulas millions of light-years away, can't we? But it's not how far away the thing is, it's how much of the visual field it covers. And the Apollo detritus is just way too small.
I'm trying to think of a good analogy to explain this. Here's what I've got so far:
Imagine you're standing on an observation point at the top of a hill at night. You can see the lights of a distant city. Maybe you can even identify some; for instance, that blinking red blob is the casino on Main Street, or that purple dot is the movie theater.
Now look down at your thumb. There are bacteria crawling around on your skin. Can you see them?
The city lights are the stars, your thumb is the moon, and the bacteria are the astronaut artifacts.
What do you think?
