Because pro wrestling has its roots in a con: working matches people thought were real, there's this never-ending "It was better when people thought it was real" debate that no other form of entertainment has.
No other form of fiction has this. Movies, TV, Books, Video Games. Kids aren't figuring out Harry Potter isn't real and then saying "WELL NOW I CAN'T ENJOY ANY OF THIS ANYMORE". No, they're saying that after they read JK Rowling's tweets. *Rimshot* *Gets hit with tomatoes*
Because for generations the people who ran and worked wrestling really did believe that if people knew wrestling was fake, they wouldn't watch. They thought of themselves, or least acted like, more as conmen than entertainers. You weren't fans, you were "marks" to be deceived.
I think a lot of old timers come from that mindset and deep down feel like if you don't believe wrestling is real, you can never get as into it as deeply as when you know it's fake. That's probably true for a minority of people but for most folks? I call bullshit.
People cry at movies, books, whatever. They devote their lives, get tattoos, to things they know from the jump are fake. All entertainment needs to emotionally invest people is to create characters you care about and then put them in situations with high stakes. That's it.
"You like ____ because they do ___. On this night they're trying to get ____ and ____ is trying to stop them. Pay money to see what happens." That's all wrestling is, that's all movies are, that's what the entertainment business is. The blanks change, the rest doesn't.
The funny thing is, there are aspects of old school wrestling that were better. In good old time promotions the storylines had fewer logic holes, the work was tighter/grittier. But the reason why is because promoters were scared that if you thought it was fake, you wouldn't come.
There's no reason why modern promotions can't have that same attention to detail. No, wrestling's not about fooling fans anymore but you should still do it, because entertainment that doesn't have giant logic holes in it that you expect fans to fill on their own is more fun.
To sum up: The fact that people know wrestling is fake doesn't lower its entertainment value or potential popularity. Bookers and wrestlers who do things that don't make sense and think "It doesn't matter, fans know it's fake anyway"? That does hurt it.