To be frank, I dont think your taxes are anywhere nearly as complicated as ours.
Maybe, but _who_ do you think lobbies for the convoluted tax laws? Who benefits from citizens feeling like taxes are "too complicated" to do without some expensive software holding their hand?
But even with the absurd deductions, credits, loopholes, and other tax nonsense, it's the IRS that is responsible for navigating, implementing, and enforcing all of these rules. They have thousands of pages of detailed explanations for how to calculate taxes already available. If anyone was in a good position to build a tax computation system free for citizens to use, it would be them (assuming of course, they were actually sufficiently funded).
Could it turn into a boondoggle like the FBI's infamous Virtual Case File system? Sure, but it might also be great for ~90% of American taxpayers, who currently pay around $3-4 billion dollars every year to prepare their taxes. So if we spent a whopping 3 billion dollars to build a tax preparation system free for everyone to use, it would pay for itself in one year. After that, the yearly maintenance costs should be pretty trivial.