I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
Ugggggh I went through this with my (boomer) boss for years until she finally accepted it lmao. Then it was, "WORRIES, CaptFeather! WORRIES!" as a joke every time I said it lol