Why not fixed budgets?

Today's quote is from page 239 of the 1960 book, The Economics of Defense in the Nuclear Age:

Charles Hitch

“With existing institutions, for example, the bargainers are often given the perverse incentive of trying to maximize their budgets (their costs) instead of maximizing capability for whatever budget they receive. A subordinate official may well feel that effort devoted to getting his budget increased is more rewarding than analysis and effort devoted to getting greater capability from a given budget.

“This tendency might be countered by some device for permitting units to keep a part of any cost savings. An extreme form of correction, which admittedly has certain disadvantages, would be to give each unit a budget fixed in amount for two (or even more) years in advance.”

Here, the principal founders (Charles Hitch and Roland McKean) of modern defense resource management admit that there was some logic to the preexisting system of very broad project discretion within a fixed budget, as opposed to make a case for as big of a budget as possible based on the requirements involved.

Why not provide fixed budgets?