Most professionals don't know the whole tax code, but unless someone owns businesses or has complicated investments, an educated 10 year old could calculate the annual federal income tax on a napkin.
If someones taxes are more complicated, pay a professional to handle them, then write your congressman imploring a more simplified tax code. Until then, free electronic filings can expect not to be audited.
For those with dead simple filing requirements but who lack a 5th grade education, free tax prep is a thing. If that's too much effort, have the employer to put "single 1" on the taxpayer's w-2 and look forward to the refund.
How did the audit go by the way? Seriously, it's inexcusable for the IRS to mess with people out of spite or to harass them over nothing. If that was systemically the case I wouldn't support more of them either.