Los Angeles Rams defensive tackle Aaron Donald is the best player in football, but he cannot be the MVP.
The MVP award might have begun as simply a catchier title for "most outstanding player," but the award clearly places emphasis on the word "value" in a way that doesn't happen in most other sports. Typically, the best player is also the most valuable, but in football, the game has placed such supreme responsibility on the shoulders of quarterbacks that they are always the MVPs.
Without a franchise quarterback, your team is almost inevitably hopeless, and with one, even the most foundation-troubling cracks are papered over.
At Pro Football Focus, we have been working to develop a football WAR metric (wins above replacement) by taking PFF grades and using mathematics to determine the correct weighting for them by how important those facets are in predicting the outcome of games. The players who do the most important things well and consistently score the highest in PFF WAR.