I've heard that this is more common with American English, and that Americans are sometimes a bit taken aback when the response is "I'm well, how about you?" or something taking the question more literally like that.
Re:I’m a very slow thinker (2016)
by PhasmaFelis on Monday August 06, 2018 @11:29PM
(#17703681)
Sometimes, yeah. In the US I've seen it go both ways, so it's not a hard and fast rule, but it does seem to be a thing.
It's not that they don't notice, it's that "How are you" can be read as either a question or a greeting in modern English.
It's even more pronounced with other phrases. If you're introduced to three people and they respond "Hi", "Hello", "What's up?", it's understood that they're all pretty much saying the same thing.