In broad terms, seeing live theater is sort of like a cross between a movie theater and a concert.
What happens at a concert? People get up for food & drink & bathroom, they have food at their seats, they overspend on merch, they photograph and video the whole thing, they arrive late if not into the opening act.
What happens at a multiplex movie theater? There are no ushers policing audience behavior, there’s food & drink, they arrive late to skip the trailers, and now people have even taken to snapping pics of the screen as proof that they’re there.
The decorum is bad, no question. But these things have all been steadily building for years and it’s not a problem exclusive to theatre.