My theory is they make it as confusing as possible so they have an excuse to audit those they want to audit. “Oh Mr. Rumsfeld, you did in fact make an error and we do believe it may have been made in past returns as well. We are going to have to set up an audit..”
I think it has grown more and more complex as would be expected with any government program.
Until, sometime during the 1980s, lobbyists began to "carve out" special exemptions which seemed to have sent the complexity through the roof.
And yes, much of that complexity is intentional, for both their ability to audit and create exemptions/deductions for their friends.