Because Congress creates really, really complicated tax laws that reduce tax burdens for business losses and obviously favor a structure that if you're wealthy enough you can exploit it for things that aren't actually contributing to our economy. Regular person with a few deductions that might go over a standard deduction? You might want some help with that, even if you're "doing it yourself" with the help of software (just like almost all of tax preparers do) or if you're just doing a standard deduction, it is easy.

Why can't you just fill out a 1040EZ in a webform and file online through the IRS?

Companies like Intuit lobby Congress so that the IRS can't just make it easy for most people. For example, the IRS wanted to just have their own app on a website to help you do your taxes for free just like TurboTax but nope.

The same thing has happened with weather in the US. NOAA/NWS pretty much provides all weather data and RADAR data to the public for free. They also provide a public API and all those weather apps either directly or indirectly get their data from there for the US.

Why don't they have an app like all these weather services do? Well companies lobbied so NWS isn't allowed to. The best they can do is provide a website that you can "use like an app." Paying extra for that up-to-date RADAR feed in your weather app? Guess what? You already did through taxes and now you get to pay a private company to simply display it in their interface.

Congress has the power to end this non-sense but does not. I'm sure this won't surprise anyone when I say this but US law is slanted towards empowering businesses and lowering accountability.