Genuinely loving everyone's deep-dives into Zoe's behaviour, but to put in bluntly she's childish. She blew up on Youtube because the main consumers of content are teenagers and she is childish.
Moreover, most people mature quite rapidly between the ages of 18-25; usually this is as a result of leaving home, going to college/university, working a series of mundane, badly paid jobs and living quite precariously. "Character building" as my Dad would say. Not glamorous but most of us grow from these experiences and come out the other side as functioning adults. Zoe never really did any of this and it shows.
Pandering to a younger audience has made Gleam millions so it's unsurprising that they never really pushed her to mature her content. All of her close friends have made their careers off her brand and therefore aren't in a position to question anything she does, enabling her to infantilise herself to her hearts content.