I used to work with a guy who had a side hustle (it was his wife’s full time gig) doing tax preparation. Jackson-Hewitt if it matters. He had purchased all the franchises in town EXCEPT for the wealthy areas: “Those people have accountants, or can at least follow turbo-tax instructions”. Most of his money was made off the in-store counters at Walmarts.
Literacy and numeracy aside, tax rules can confuse even very educated folks. Very few people who have accountants do their taxes are dumb, after all. If you struggle to read, or with basic arithmetic, then doing your own taxes is out. If you struggle to read, or do basic arithmetic, then you probably don’t own a computer, so turbo-tax is out. And has been mentioned, many of the tax credits deductions, etc. available to poor people are even more complicated than what the middle class deal with, and these are the people who have the most trouble understanding the rules…so they really do need help, and come out ahead even if the help they get is rather mercenary.
Aside: That guy I worked with was pretty good at stretching a penny 'till Lincoln screamed. At the end of the year, he would sign up for zero-interest credit card promotional offers. These he would use to buy a bunch of cheap used computers off ebay. These would be used for the tax offices, with minimum payments made on the credit card accounts. At the end of tax season, he would put all those computers back on ebay, and often make a profit there, and thus pay off the zero-interest credit cards, which accounts he would then close. If for some reason he lost money on the computers, that was a business expense to be written off.
This left rent and his franchise fees as the big expenses. The franchise fee got him advertising and software that allowed cheap fairly unskilled workers (data entry, basically) to do the preparation. It was a pretty sweet deal, and he was only keeping the day job for the health insurance.