When Ronald Reagan proposed a complete re-write of the tax code in 1986, his fellow Republican and head of the Senate tax-writing committee, Bob Packwood, responded: I kinda like the tax code just the way it is.
The tax code is used for a lot of things that have nothing to do with taxes. To help poor families, to promote alternative energy, to build bridges, to promote diversity, to grow food, etc., etc. That's a major reason why it's as ungainly as it is. It's also true that it is filled sith special interest provisions protected by often just one US senator or powerful congressman. Many of these do benefit particular interests, often businesses who profit from them but also provide jobs for constituents of the representative protecting the provision. Scoop Jackson was not called the senator from Boeing for no reason. He protected Boeing jobs like a lion.
This does not, to be sure, result in a set of laws that are completely rational. Nor does it mean there ought not be reform. Just trying to explain why that reform is so hard to come by. I would bet that the military commands would cut at least a third of the Defense budget because it funds stuff they don't want or need. Take a look at the VTOL Osprey aircraft for example. It falls out of the sky with frightening regularity and has no obvious mission, but contracts to build it are spread across dozens of states.