Earlier quoted context omitted.
Because it is trivial to do, and regularly required? Would you say they don't need to know how to change a tire too? Add more windshield fluid? Wash their windows by hand? Open their glovebox?
Being able to change a tire is a great skill to have because it may be very beneficial for safety. The rest are blatantly obvious things that nobody has to learn nor devote time to doing while they could be doing something else. Well, except maybe for adding windshield fluid… but they do that for me when they change my oil. :)
To me, changing your oil yourself is somewhere on par with being able to connect to wifi yourself. Sure, you'll often just get other people to do it for you when you are having other more extensive work done on the car, but having to get somebody to do it for you seems sort of like having to call "Geek Squad" to connect to your home wifi. Far more prevalent than you would hope or expect.