Natural processes. Mountains erode, plains build up. The earth tries to achieve a level surface (lowest energy state) but keeps getting set back by mountain buiding (Himalaya, Alps etc). As mountains erode some particles are blown away and they have to settle somewhere. So the archaeology gets buried. Same as the dust in your house - if you don't hoover it up eventually it looks like Bleak House.
There were no binmen so waste got thrown to one side and where people lived crap accumulated
These.
Also, in some cases rubbish was buried to dispose of it anyway, in just the same way that we put stuff in landfill now. Only instead of whole quarries, people of old would chuck their rubbish in smaller scale pits, maybe those leftover from excavating clay for pots, or the old foundation holes of destroyed timber buildings.
But also, natural processes. After the Romans left, a lot of their abandoned villas and buildings were seemingly deliberately avoided once the useful stone above ground had been robbed out. It wouldn't take long for a layer of dust and leaf mould to build up. Think how autumn leaves build up and form a mush if you don't clear them. Then add in animal poo, dead animals, and plain old dust. Then think of 100 years of that process. Then 500 years, or a 1000. Once a place is hidden, life goes on above and around it, and the debris thrown away gets added to the mix... Soil is just the leftovers from life, once it's been chewed by worms.
All of which means, I spent a lot of each summer for three years, in a hole! (And to my mind, a rather dull hole at that, but never mind.)
(And Asterix is right, there is lots of stuff left above ground too. Now, even the remains of WW2 and the Cold War can be looked at as archaeology).
