It is more complicated than that. Often it is about making sure X is taken care of and you are responsible for X. Those typically are in the higher tier pay rank than just show up and work X hours a week. In those jobs typically you have "emergency" that you need to work weekend for and then afterward the boss let you take some paid time off.Exempt means salaried, and a salary is based on X amount of hours a week. Pretty simple...you pay me for 40 hours, so I will work 40 hours (normally). Additionally, if you treat your employees right they will gladly go the extra mile for you...I don't mind the occasional Saturday but it means I just earned comp time to take later.
If they abuse that people will start leaving, words get out, and nobody wants to work for them again (unless they get paid huge salary like FANG).