Bingo. I can’t think of any other EU/NATO/FVEY country that actually does development of defence tech on the scale the US does.
That doesn't mean the US gov can't find auditors with proper security clearance to account for those black budgets.
You’re missing the “compartmentalized” nature of TS/SCI projects. You would need many, many auditors or the paperwork would be enormous to be cleared for each one.
Yes it’s technically possible, but the current model doesn’t support moving between compartments easily.