Many people fill out the 1040-EZ themselves (a one-page form you can use if you just take the standard deductions and have no unusual tax situations). However, if you can deduct more than the standard deduction, you have non-trivial investments, or any of numerous other situations apply, you have to fill out the full 1040 form, and that requires sufficient effort that there's a substantial tax preparation industry in the US.
That said, there shouldn't be; there's quite a bit of lobbying by the aforementioned tax prep industry to prevent simplification of taxes to the point where everyone could simply file them on their own, as well as to prevent the establishment of an online government-hosted tax prep system. And that's completely ridiculous. The IRS already has more than enough information to simply send everyone a pre-calculated bill.
That said, there shouldn't be; there's quite a bit of lobbying by the aforementioned tax prep industry to prevent simplification of taxes to the point where everyone could simply file them on their own, as well as to prevent the establishment of an online government-hosted tax prep system. And that's completely ridiculous. The IRS already has more than enough information to simply send everyone a pre-calculated bill.