Davesport; when a roadie approaches another cyclist, a quick decision is made;
they'll look at you (what else is there to go on?) and decide if you look like a 'cyclist' or just a 'person on a bike'
a 'person on a bike' will get ignored - in much the same way a car driver will ignore other car drivers.
a 'cyclist' will get a VERY subtle hand twitch / micro-nod / etc.
this is not out of disrespect to the 'person on a bike' - but there's simply no point acknowledging everyone they see on a bicycle, especially when there's a good chance the 'p-o-a-b' simply won't notice them.
you said yourself:
Davesport - Member
...every single one of them roadies I encountered. Eye contact then the head drops & stares into the Cateye speedo / Garmin GPS...
there you go, that was the acknowledgement you crave.