TurboTax and HR block lobby considerably to prevent the government from making the process of paying taxes free, fast, and painless for the overwhelming majority of taxpayers.
https://www.propublica.org/article/filing-taxes-could-be-fre...
We can argue that the tax code should be simpler but that's a different argument.
Countries with sane tax collection systems make it easy for you to add those deductions, either on paper or with a web service run by the government, for free.
And your mortgages, unless they are from international banks, yes, the IRS already knows about those too. Let me put it this way: if it goes through a US domiciled bank, the IRS knows about it already.
Yes, people with complex tax returns do use accountants to file their taxes in countries with sane systems too, but the overwhelming majority don't need the complexity for the equivalent of a w2 and/or a couple of 1099s.