While obviously winning the game is the most basic objective of any sport, one must keep in mind that when the sport is a team sport, with uneven schedules, with injuries and therefore personnel that changes from game-to-game, inconsistent coaching decisions, poor refereeing, the skill of the field goal kicker, the hands of receivers and so forth, the end result of any given NFL game isn't necessarily in the hands of the quarterback, yet he is the onestuck with the win or the loss in a particular game. The difference between a win/loss record for a quarterback in football as compared to a pitcher in baseball, is that starting pitchers are in rotations, so that it isn't that difficult to compare a starting pitcher's record with or without him in the lineup on the same team and therefore come up with a statistic indicating his overall worth, which while still being somewhat suspect, at least tries to measure his material worth with some meaningful success; whereas in football, quarterbacks are not rotated in and out, unless they are deliberately benched or injured, so it's difficult to fathom what a given team's record would be with or without a particular quarterback, so that a good quarterback on a bad team, will probably have a poor record, and a fair quarterback on a really good team, may have a very good record, to which it is unknowable as to how each would have fared had they been on each other's team.
The problem with quarterback win/loss records is that quite obviously they pretty much reflect the exact win/loss record of the team that they are playing for and since quarterbacks are only one factor in the winning or losing of a given game, there will always be some quarterbacks that will have records that far exceed their true ability and other quarterbacks that will be shorted in their real ability, if one were to look exclusively at only the winning percentage of that quarterback. This doesn't mean that a win/loss record for a quarterback doesn't have purpose, but it does indicate that it doesn’t really have a lot of purpose or that it is particularly meaningful.
On the other hand, the head coach is responsible for pretty much the whole gamut on the football field, so that his record on the field of action is meaningful. In actuality, with the exception of the fact that the head coach is not a player, his decisions more than anyone else playing the game, influence whether the team wins or losses on a given day. It certainly seems much more equitable and more logical to stick head coaches exclusively with win/loss records, but far less so to pin it additionally on a quarterback.
The fact of the matter is that there are so many other meaningful statistics that will validate whether a given quarterback is any good or not, that the winning percentage of a given quarterback will almost never shed any additional light on his skills on the field, so that there really isn't any good reason to attach wins to a quarterback, especially considering all the other factors and players involved in a given game.