Let's look at what soil build up is caused by.

Soil increase or degradation is interesting. Some places gain soil over time, others they lose it, but both tend to happen very slowly. There are generally considered to be 6 types of soil bases: sand, silt, chalk, clay, peat and loam. Loam and peat are organic in origin, meaning they are at least partially created by decaying bio-matter. These take a lot of biomass to create, so depth in a few thousand years on a plain would not change much. Chalk also has its origins in organisms, and it takes millions of years to form from the build up of marine skeletons, so that couldn't form since stone henge was constructed. Sand silt and clay are essentially different particle sizes of rock with different properties. These are deposited either by wind or water through erosion of rock, again over millions of years. The plains in Wiltshire do not really fit the criteria for rapid soil deposits . A soil analysis can be found here

https://brian-mountainman.blogspot.co.uk/2011/04/soils-of-stonehenge-area-no-trace-of.html

So, by this logic only things we should be excavating are as a result of soil build up, and are thus either: near water which have silted up like the Roman Baths in Bath, or things buried in the sands like in Egypt, or things like Pompeii, covered in volcanic ash (which are near unique in history). So why do we keep digging things up which don't fit these criteria?

Because the tops were demolished and only the foundations remain, and these are what you are actually looking at.

It's probably worth pointing out that there are hundreds of other megalithic monuments in Britain and none of them are buried either. In fact, I can't recall a single example of a megalithic site in Europe having to be excavated (though probably somebody will be along to prove me wrong shortly!)