If the USA didn’t have such a complicated tax system
For 95% of the public, its not complicated. Its just getting all the independent pieces of information from different private agencies.
W2 from employer
1098 from your mortgage company
1099 from your retirement account firm
Prove you have health insurance
Prove you have student debts
Prove you have a small business and you've tracked your receipts
Prove you have children
Prove you paid taxes to your state
Once you have all the numbers lined up, its simple arithmetic. Easy for a computer to do.
But knowing who to ask for all the individual chunks of data is an obnoxious chore that only one organization does particularly well. And that organization - the IRS - won't tell you the information they have. They want you to guess and tell them what you have, so they can tell you if you got it right or not.