I'm not suggesting that the federal system would print out the state taxes complete, I'm saying that just having access to the federally prepared information would greatly simplify the state filing.
Like my taxes are relatively simple, so my state taxes are just a few calculations using my taxable income from the federal 1040, and then also I have to attach any federal schedules I filed. Having the completed 1040 and printable schedules would reduce the state filing down to a few minutes of work.
If that solved people's problems, then there would be no need for people to buy tax prep software that does state returns... and some don't, but it is an extremely popular selling point of TurboTax and similar software.
My tax situation is not complicated and my state filing is 11 pages, plus 8 pages of worksheets. Federal schedules are not accepted, my state has similar but slightly different counterparts.
Like my taxes are relatively simple, so my state taxes are just a few calculations using my taxable income from the federal 1040, and then also I have to attach any federal schedules I filed. Having the completed 1040 and printable schedules would reduce the state filing down to a few minutes of work.