There is another reason: a group of Republican lobbyists (Americans for Tax Reform) deliberately want to make taxes more difficult. The idea is that 1) if taxes are difficult, you will spend as much as your time trying to get every deduction and 2) a prefilled tax form from the government would look too much like a bill, so people will pay it without contesting.

Of course, TurboTax/Intuit also lobbies to keep taxes more difficult to protect their profits.

https://www.washingtonpost.com/blogs/post-partisan/wp/2015/0...