At the risk of stating the obvious . . . buses run for the benefit of passengers, or more precisely when passengers are likely to need to travel. on Mondays to Fridays, that's to work, to go to school, to shop. Timetables are built around those times, and a certain number of drivers and buses are required. On Saturdays, the school numbers aren't there, but the rest are. Timetables are subtly different for that reason. On Sundays, shops are open for a reduced period, and timetable hours are shorter for that reason.
Stagecoach timetables are good examples of this . . . especially on a Sunday; buses start around 0900 and finish around 1900 . . . quite simply, there is almost no evening demand on Sundays, and finishing early is the way for all drivers to be available on a Monday morning!
If the bus itself is paid for from the MF work, then usage on Saturdays and Sundays only has to pay for the driver and the fuel . . . with reduced hours of operation that usually works out. For a commercial network, that's the way the planner will work.
For a tendered network, the sums are different . . . each route has to be more carefully looked at. In the case of Dorset, in about 2012, the network is completely tendered, and if each trip carries less than about 10 passengers per bus hour, then it may not survive. Their network was cut back to MF only about 2019, and is only now coming back (in a limited way) on Saturdays.
I'll also make a final point about Saturday frequencies . . . if the the service level is the same, then more drivers will be needed to work . . . and nobody (seemingly) wants to work at the weekends! A reduced frequency needs fewer drivers to work . . . which aids staff retention . . . look at the Brylaine example in Lincolnshire; they cut weekend work altogether as it meant they could resource weekday work reliably.
There is no one answer . . .