The statutes of the Nobel Foundation, ยง 4 reads (in part and translated): "If a work that is being rewarded has been produced by two or three persons, the prize shall be awarded to them jointly. In no case may a prize amount be divided between more than three persons."
Agreed, but it's in the statutes, so the board has to abide by them (or risk liability). It's also non-trivial to change the statutes of a foundation ("stiftelse") in Sweden.