For many it is not a matter of cost.

I do my own taxes as (a) I want to know what was done, since I am signing the return and have the legal responsibility, and (b) I can do a better job than most CPA's would do, since I know the records and I have the motivation... sorta like why I do my own auto repairs when possible, rather than depending upon a dealer or mechanic. Frankly I could never fathom why anyone would hire a CPA to do their taxes and put their financial fate with the IRS into the hands of a complete stranger.