Where are the meteors?

This might be a bit of a stupid question, but when a meteor/asteroid impacts the earth or moon, where does it go? Why is there no big hunk of rock or metal left in the crater? And also why is it that almost every impact crater is a circle with a flat bottom? Shouldn't the impact leave more of a concave effect? Why is there no trail from the impact (if some hit on an angle? I thought perhaps over the years the crater simply filled up with dust/debris etc. but even the ones that are on the moon are the same, circle with flat bottom, why is that? and where does the meteor go? does it simply disintegrate upon impact?