So what are they? You can comprehensively quantify EVERYTHIGN that goes into wins and losses? Seems kind of a random thing to me...that you can't always pigeon hole.
It really is generalities, really. Great defense. A couple lucky calls. A key turnover. There's just so many things. That boil down to...
did you win or lose.
You cannot and that's the point. We do know the variables for the most part though and quarterback play is only a subset of one of those variables.
It also comes down to a holistic and reductionist approach to things.
Reductionism has led us to chemistry and physics where we can predict the motion of planets engineer cars and computers and all manner of accurate and precise things.
You reduce the quarterback into the stats of yardage yards per attempt accuracy touchdowns picks sacks drops and so on and so forth.
On the other hand you have wholistic analysis which describes sciences like psychology or economics. Human behavior and markets remain wildly unpredictable because of the approach.
At the end of the day looking at wins and losses to judge quarterback play ignores the performance of all of the other positions on offense, the defense and its constituents, special teams role, and all of that plus the quarterback for the other team.
If you reduce it to yards accuracy etc then you can encompass most of what a quarterback actually does in a game. of course there are intangibles and things that are hard to quantify like leadership pocket awareness capability and so forth however in Prescott's instance by all accounts he excels at all of those.
When viewed with a reductionist scientific approach Dak Prescott is a very very good quarterback.