It's Grover Norquist: https://www.huffingtonpost.com/grover-norquist/grover-norqui...
To give these folks something they don't deserve (an assumption of good faith, non-cynicism, benefit of the doubt, etc): they think "letting" the IRS do folks' taxes will lead to more taxes and poor, defenseless taxpayers simply won't have the courage to challenge a "bill" from the IRS.
Simple fix: let folks contest whatever final "pay this number and you're done" amount they get invoiced for & sear that into federal law. But you probably won't ever hear Grover and all the other looney tunes ever mention that.
They don't want easy, hassle-free IRS filing. It's an opportunity for people to be pissed (who wants to pay money for the government)
There's also talk about your taxes fitting on a postcard and that, as usual from the scumbags nominally in charge of our country, is bullshit too because it just conceals a ton of paperwork (source: https://www.accountingtoday.com/opinion/the-postcard-tax-ret...)