Putting my amateur psychologist's hat on, this is my take (base on absolutely no inside information whatsoever!)
I think they were together from childhood and had a great time as teens. I imagine the two families are very closely intertwined. Her parents are friends with his, his sisters feel like hers etc. It is all very familiar and very hard to remove yourself from that even when your partner is being unfaithful.
From her perspective: Without him she is nothing. Her YouTube following, her influencer credentials all come from being linked to him. If she loses that she is toast. Of course she could get a proper job but I don't think influencer types think like that. So, she puts up with the cheating in return for the career, clothes, money, holidays and nice home.
From his perspective: it's easy, familiar, it keeps the two families happy and they are probably very good friends.