I'm not going to be ageist here because, as folks have already said, there are curt and brusque responses from all ends of the age spectrum. One person's impolite may be another's "efficient and to the point." And yes, I also find myself bristling a bit with some of the more curt queries and responses.
That said, a few things to consider:
The median age on this forum tends to be higher than, say, on Reddit - such is the typical viewer of PBS and Rick Steves' programming, in general. Nothing wrong with that, but it sets a basic bar against which to grade things.
But I'll put it this way: don't judge a book by its cover.
Some folks only ever use a smartphone for their internet access (or at least it's their primary means of access). Typing longer, more complete sentences with fingers and thumbs is not easy! This is how the new lingua franca of younger generations happened: once typing became tougher, sentences were truncated out of basic practicality. Does this mean the people communicating in this way are mean or impolite? It's impossible to say, to be completely honest.
I'll admit that I'm not always the most forgiving of some things I read here and on other travel forums. But I'm more likely to direct my criticism directly to the offender via direct message. Or, more often than not, I'll simply not say a thing. Sometimes it's not worth the bits and bytes.
The RS community is a wonderfully informed group that definitely has its strengths and weaknesses - and that's what makes it fun to be part of!
Cheers and good vibes to all!