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.
A few years ago a very boomer gen-xer tried this on me and got very enraged when I would say "hey" instead of "hello {his name}". At one point even threatened me.
I believe it. I have seen them melt down over the tinest thing. You can shorten my name, like pretty much every first name, and one of the old shits I used to work with would scream and yell about it.