Now to qualify I am not a Geophysicist or a Geologist, but why...

Now to qualify I am not a Geophysicist or a Geologist, but why would the motherlode be way deeper?
My understanding is for the diamonds to be in the alluvial channels it must have reached (or near to it ) the surface.
Potentially it has all been washed away into the alluvial channels or potentially covered with massive amounts of overburden.
The fact that many kimberlite pipes (some diamondiferous) have been located at or near surface would suggest it is likely that the motherlode would be in a similar position. It may be from a different historical time period than those recovered and suffered a different fate but I don't see any reason for it to be impossible?
Happy to be enlightened!