As primarily a roadie, I always say hi, whether to MTBers, other roadies, or pedestrians. Mostly because I like being nice to people, and find that they tend to be nice in return.
The only exception to this is if I am going swiftly downhill, and my eyes have watered so much, I have to concentrate more on staying alive and upright.
ERGO, I don’t know what the OP is on about.