For the same reason why in most jobs you usually don't work for more employers spread out over your week. Like, 2 hours on Monday for Alice and 3 more for Bob, then Tuesday is your Carol-exclusive day.

Hiring people is costly, and if everybody works fewer hours you need to hire more people to get the same output. More people also means more communication and management overhead. So suddenly changing your scheduling approach is probably net negative in the short term, if other businesses continue with the previous workload.

If most of the population agrees that 5x8 is reasonable work hours, then that's what employers pursue with scheduling. If you pursue to change the status quo (say, 3x8 or 5x6), then you should be ready for people who still think that 5x8 is perfectly reasonable and fine for them so they are going to continue doing it their way. And to the market which may not adjust to lower individual output during a significant part of your lifetime.

Of course it the entire humanity just woken up tomorrow and everybody decided that it is enough to work only 3 days per week and anything more is slavery, then the scheduling will just adjust and more people will be hired to fill the time slots.