Because customers tend to want to map why the Ferrari they asked for is a Fiat with bandages, given the fixed price budget, and thus want to map each requirement to the work time spent on it, and expensed budget.

Then comes all the argumentation why all those hours were required for each item, due to project delivery challenges on either sides, decisions that would require even more work, needless efforts spent in stuff we shouldn't have done as part of fixed price, and so forth.