Depends on the feel, eye contact, etc.  I normally at least say hello and have a quick exchange if they seem amenable; I have only had one group ever not respond in any fashion.  Boaters in general, whether on the ocean, a bay, a river or a lake, will exchange at least a wave at a distance to acknowledge one another and often a verbal exchange in closer proximity.

Your list of hypotheses could be accurate or completely unrelated to the situation but is purely speculative.  There seems to be a bit of bias in several of those such as age, experience and area of residence...you could be missing out on interacting with a number of wonderful people with valuable insights if you allow those generalizations to limit your engagement.  I'm not implying you are bigoted, but those factors are ones we should all be aware of to limit their influence on our judgement upon meeting or working with others.

That kind of thing is always hit or miss with people who want to engage and people who don't, plus the COVID thing I'm sure is a factor.  I fall into conversation with other travelers at times, but sometime they'd rather not, and sometimes I'd rather not.  For me, during those "rather not" times, it is generally because I'm feeling the fires lit under my heels to keep it moving on a travel day.