I don't think it's a Republican thing, the Liberal party and the NPD in Canada are both left-leaning and they have a majority yet they aren't pushing for this.
Pretty sure it's just run-of-the-mill lobbying and corruption unfortunately. A typical "think of the jobs!" type of thing.
What parent is referring to is not just lack of pushing for it, but active campaigning against automatic filing. Not a "think of the jobs" but "automatic filing is intrinsically bad."
Republicans and the Liberal party in Canada are both neoliberal when it comes to their economic policies, they mostly only diverge on social issues and social services.
It kinda already had been addressed. SimpleTax was released as donationware and the CRA introduced NETFILE and Autofill. Filing a return today is way easier than in, say, 2011. Buying TurboTax has been unnecessary for a decade.
Pretty sure it's just run-of-the-mill lobbying and corruption unfortunately. A typical "think of the jobs!" type of thing.