I would suggest that legal professionals are, let's say, not immune to fallacious reasoning or category errors.
Lawyers are also trained to make the best arguments on behalf of their client. They have to think they are properly arguable but they are used to picking a side, with a desired outcome in mind, and doing their absolute best to get there within the bounds of the law. It's different training to, say, the ideal scientific training which is to stand neutral, observe the facts as they present themselves and reach the best conclusions based on that evidence, no matter where it takes you.
Added to that, professionals (infusing but not limited to lawyers!) - who are likely to be very used to being thought of by others and themselves as really quite clever - may well find it harder to make serious allowance for the possibly that they have made fundamental mistakes in their reasoning over they have committed to a path. Hence the arrogance, as you say!