How would an eager startup person get funded to build something that competes with TurboTax? How will you bring novel value to the industry at this point?
A monopolized industry is also intentionally difficult to enter. It’s not just about buying existing competitors it’s about also making it as hard as possible to enter.
So even when some startup manages to get funding and deploy a viable product, they’re immediately on the radar for acquisition for monopolists if they weren’t before going to production.
TurboTax has also lobbied to affect US law in their favor. They’re a scumbag organization who makes an excellent product.