The why is by design. Politicians in the US's first past the post voting system are there to enrich themselves. They enrich themselves by making sure that the people who paid for their campaign are getting the appropriate tax structure and subsidies they paid for. Because state level budgets are finite, anything that does not advance the goal of the donor's is optional. In addition think tanks such as the heritage foundation, or the third way, or hundreds of others that are the same with different names, are on a mission to make sure that government is privatized. It's a slow multi-decade process, but the post office, the TSA, security clearances etc etc are examples of their success.
It's very easy to make government agency's efficient, but making an efficient government means that a middleman doesn't get paid, and thus in our hellish system, an efficient government cannot exist.