I think the comment has everything to do with it: every time you use the tax code to legislate the behavior and choices of the people, it causes the tax code to become less simple by the very fact that the citizen now has to track that behavior. For example, take charitable donations. Because they're tax deductible, I have to track them, or forfeit the deduction. That's extra, manual work that must be done. Now multiply that out for every behavior that some congressman thinks should be encouraged, and it shouldn't be surprised that the taxes are complicated.

A tax form is essentially a giant function, but one with tons of inputs. It can only be automated once all the inputs are known (essentially, that's the job of tax software). The only person that has all of those inputs is you, so you're forced to buy TurboTax to be able to reasonably input and evaluate taxes.

If you're thinking that all these inputs could ever be tracked by the federal government in a giant database, I still think it's crazy: you'd need a database for student loan interest paid, which every loan organization reports to, for charitable donations, for stock purchases, for stock sales, for dependents, etc. (I don't even know how you'd begin to track dependents, aside from how we do it now: manually.)