Turbotax, H&R Block, and the other tax companies are massively wealthy companies that actively lobby to push laws to keep them from being simple. There are states that they have managed to bar the same thing from happening in.
They are actually not the de facto for large company when it comes to payroll. I think the majority of their customers at company’s with less than 10
employees. I use to work for Intuit on the Quickbooks payroll product. I left about two years ago but I don’t think it’s changed that much.
The other guy linked the answer, but I'm going to explain it anyway:
In the US, companies have the freedom to bribelobby our congress members by giving them money that's totally unrelated to their vote you guys. The reasoning behind that being ok is that the congress official in question is still technically free to vote however they choose despite the money given to them. The reason lobbying works is the threat that the congress person might not get that money next time if they vote against that company's interests.
Just so you all know, because our congress members make a government salary of about 150k-250k/year, it's surprisingly cheap (from a rich company perspective) to lobby them, with lots of payments being in the low thousands. So for obscenely wealthy companies (like intuit), it's much cheaper to pay just enough guys off to kill a movement than for them to suffer the actual consequences of that movement.
In this case, intuit's entire business model depends on American taxes seeming like this mysterious and unapproachable thing that Americans have to pay a third party for in order to not get thrown in jail by the IRS. And given that intuit (and companies like H&R block) rake in billions each year, it's comparatively pennies to pay off congress officials to keep it that way.