We compare QBs because QBs are comparable. It's really difficult to build a narrative around the number of passes one person completes vs the number of tackles a different person dishes out to different people on different teams, but very easy and meaningful to compare completion percentages for two quarterbacks. It's also because QB is considered a leadership and decision-making role moreso than any other position on the field, and so the overall performance of the offense tends to be attributed to the team as a whole but the QBs are compared on the decisions they make on the field and the rest of the offensive units are compared on their ability to execute those decisions.