Much of that overhead is not going to admin salaries (although, as stated elsewhere in the discussion money is fungible) but covers things like the cost of buildings, labs, maintenance, etc.
I had no idea the on-going costs of physical things did not decrease over time, what an interesting thing to know. Here I thought that after you build the building and paid off the building, then for funsies also bought all the land surrounding the building, that at some point it would drastically cheaper to maintain it; but what you're saying makes so much sense!
Turns out, this is also research grant money. Half or more of every grant usually goes straight to the university.as "overhead."
The universities could change this so more finding went to researchers, but they have zero incentive to.