The short short version is that Intuit and H&R block make software to make filing taxes easier, market heavily and lobby heavily to prevent the IRS (which is itself a separate entity from the federal government whose sole customer is the federal government) from making similar software

For most people who have a job and rent the place they live they can fill out all of their taxes in just a couple of pages of forms (see the 1040EZ for example. Just punch in the info from the W2 your work gave you, some quick addition and subtraction then mail it off) and the IRS already knows most of the information, so it's already extremely easy, and if you own your home or want to take advantage of additional tax incentives it might be a bit more complicated. You can always download the forms and do it all manually, but tax software generally makes it easier