Very salty answer because it's 8 am.
Why should a plumber go to school? Why don't they just learn everything there is to know about plumbing by looking at a toilet?
Why should a welder go to school? Why don't they just learn how to weld by looking at a torch?
Why should a pilot go to school? Can't they just figure out how to fly from looking at the controls?
Why should a vet go to school? Can't they just figure out how animals work by looking at them?
Yeah, in a SHOCKING twist of events hairstylists don't get hired by walking into a salon and getting looked at by the owner who tells us "well you look like you could hold a pair of scissors. You're hired!!"
Also. Hairstyling is a fucking trade. Just like plumbing and welding and pipefitting. There's a lot that goes into it, despite the majority of the public thinking what we do is so easy they can do it at home and that we're too stupid to chew gum and breathe at the same time.
How about you go hand your friend who doesn't know shit about hair some crafting scissors and ask her to give you a cut like this picture you saw online and THEN you can come and ask me "why do you need to go to school to cut hair?"