Because that's what the LRN nav 3-1.30.20 section says your supposed to (FM 756 page 1401).

Record progress page fuel - not fuel off fuel gauge - due to temperature variation of the the fuel density and variation in gauge accuracy.

(From Page 1401):
Note: There is normally a relatively small difference between the calculated fuel onboard (as determined by the FMC/fuel flow inputs and displayed on the POS REPORT page) and the sensed fuel (as determined by the fuel quantity indicating system). This difference is normally not significant, and is the result of small inaccuracies in the sensors, or changes in fuel density due to cold soaking. Unless there is substantial evidence to indicate a gross error in the calculated fuel value it should not be altered, and this is the value that should be logged on the MFP.