I think it's a result of legacy stuff (i.e. "it's worked like this forever, why change?") and a historical and cultural distrust of the government which is far more pronounced and has had a much larger impact on how people interact with the government than in other countries. People in America tend to distrust the government by default in a way that can be baffling to people from other countries. They want to do a lot of stuff themselves.
Also, for-profit tax filers have aggressively lobbied to combat simplifying the process for decades.
Also, for-profit tax filers have aggressively lobbied to combat simplifying the process for decades.