An interesting question and one I'd never considered.
Perhaps it developed historically. In the beginning, one either got out of the field .. or didn't, as the case may be. Although one sees elementary performance data in the post war period, this hit and miss technique largely continued up until the ICAO PAMC (provisional acceptable means for compliance) relating to performance came onto the scene around the time of the F27's introduction. I can recall tales from Centaurus along the lines that he used to have some concern with military VIP flying in the pre-jet days where it was left largely up to the commander, somehow, to get it right ... Even now, DC3s and the like have only the most rudimentary performance data (unless things have changed since I last played with a DC3).
The PAMC revolution then developed into the normal extensive certification performance data we have become used to over recent decades.
So far as the term "scheduled" is concerned, I suggest that the Industry arrived at that term to reflect the notion that the weight (mass) aspects of the conduct of operations were constrained by predetermined calculations. These provided predicted maximum weight data for which operations would reasonably be likely to meet the relevant design and operating standards .. hence the idea of scheduling in a manner similar to generating a timetable or other source of predictive reference data.