friendly conversational etiquette
And many, perhaps most, are first time posters. You say "Hi, welcome, bla bla bla". You give 'em an answer or a perspective or whatever, as do others... and then, nothing.
I think, aside from a lack of search skills, that, at least with the new ones, they lack any forum social graces or have no interest in participation... other than obtaining answers to their questions. Who's to say?
Perhaps a rule about 1st timers doing something other than asking a question would be a prerequisite? I have no idea how that would work, short of mods and members policing the posts.
It doesn't drive me crazy, but it certainly is annoying.