Because companies like H&R block and TurboTax make a lot of money on it. For most people their taxes really are simple enough for what you say to work and be cheaper for everybody. In fact the IRS already has all the numbers for most people to do that, or if they don't you have an accountant to handle your finances anyway who can quickly give the IRS those numbers. However by making you enter all the numbers and calculate things it gives the third parties a reason to exist.
I used to do my taxes by hand: it was actually faster than the online system I use now - mostly because it was a lot quickly to skip the section dealing with rail road retirees and such. I quit doing it only because one time I missed copying line 13 of form 1234 to line 47b or form 5678 and got the wrong numbers. A simple mistake that the IRS caught right away, but then I had to go refile my state taxes which depended on the wrong numbers: that was painful enough I quit doing it by hand. (why the state didn't also catch the mistake and avoid the whole refile garbage I don't know)
I used to do my taxes by hand: it was actually faster than the online system I use now - mostly because it was a lot quickly to skip the section dealing with rail road retirees and such. I quit doing it only because one time I missed copying line 13 of form 1234 to line 47b or form 5678 and got the wrong numbers. A simple mistake that the IRS caught right away, but then I had to go refile my state taxes which depended on the wrong numbers: that was painful enough I quit doing it by hand. (why the state didn't also catch the mistake and avoid the whole refile garbage I don't know)