Like any other programming language, by definition, 50% of programmers are below average.
If 70% success is a "passing" C grade, that means you have a 7 in 10 chance of choosing a programmer of failing quality. In other words, your odds of picking someone at random is biased towards failure.
This holds true for almost anything where you are choosing a skilled worker at random, including tax preparers.
Unfortunately, most people do not have the knowledge or expertise to adequately interview tax preparers to increase their odds of picking one that is in that 30% passing grade side of the curve.
And, I think, many people in skilled trades get by or even rely on that fact in order to stay employed in their line of work.
And that's why I continue to do all of my own tax filing, because, for me as a programmer, the US tax code is nothing more than the source code for a program written in its own language which happens to be a special dialect of English.