No way would I ever trust open source tax software not to mess things up somehow. The paid software has a big incentive to get it right.
It's relatively easy to screw up your taxes by misentering a number into the paid software as well. The various guarantees the paid software people offer don't cover this sort of situation. I don't think this is really a factor.
Which is probably not the kind of mistake you’ll have on your taxes anyway. Basic arithmetic is easy. And if you do make these mistakes it’s not likely the IRS does anything more than correcting it. But it’s more than no guarantee.
Also they do provide human support.
There’s nothing about paid software development that requires source code to be kept private.