It's probably more-so that above a certain income threshold, your tax return becomes exponentially more complex.

Free-file is useful when all you have is 1-2 above-board wage earning jobs, maybe with a 1099 contract job where the employer is nice nice enough to pre-fill the form out for you.

Once you start dealing with non-trivial amounts of dividends, interest, securities, partnerships, real-estate, you really do need a professional to keep things organized.