They can for the majority of Americans. Congress also has the ability to mandate more reporting, and more reporting from more agencies would pretty much have to be part of a 'simple tax return' bill.

The point never was "they do prefect tax returns for everyone." Everyone knows this isn't feasible.

The point is, they'll simply send you (web) forms prefilled with the information they already have, you go in and add information they don't have if something is missing. So even if they have incomplete information, you'll still benefit greatly. If youre feeling really upset about the whole concept, you can tbrow away everything they prefilled out and download a blank form and do it yourself from scratch and that would be perfectly fine too.