Because Jackson-Hewitt and H&R Block asked them.
Bingo.
Most of the Jackson-Hewitt and H&R Block tax preparers are independents, hired for the duration of the tax season. Folks can get online training pretty cheap between seasons, and tax law is so complex that it is not unreasonable to require some effort to stay abreast. An error on your tax return can costs you hundreds or thousands due to a deduction you failed to take, a credit you are allowed but unaware of, etc. And since most returns are not audited, you won’t find out about the error.
Same with computer tax programs. They work OK, but the software I used last year prepared my state tax return incorrectly. That wasn’t entirely its fault, the tax exemption WAS programmed in to it, but it wasn’t in an obvious place. If I hadn’t caught it on my own final review, I’d have paid too much for my state taxes.