There is not a government agency that does not need a serious haircut.
However, I believe the persons most responsible for the size of the government vehicle fleet are the congresscritters who have domestic auto manufacturing within their district.
GSA buys alot of these vehicles as pork and pushes them out to agencies. The congresscritters often provide a list of vehicles (usually those they can’t sell) to government agencies to “order”.
Much of it is good old fashioned pork.
If the GSA needs to buy the vehicles as pork, I think they should send me one for my use. I promise not to drive the vehicle except when I’m working for the government, which is about half the year at current tax rates.