It not just you i have noticed it in the last couple of years, it doesn't bother me let them get on with it, i was out today and had a good lot of nods and waves and the ones that pretend they haven't seen you.
If they are on adventure bikes they won't even have seen you .. to busy texting or fidling with the sat nav.. harley riders will be to busy playing with the stereo or ashtrays... ducati riders won't acknowledge you if your bike is worth less/more than theirs... dem r1 boys would rather stab you and other Japanese bike riders are scared you might be one of dem r1 boys... Strange times
I personally think it's always been like that in my time on the road,a 50/50 chance a fellow rider will respond,but these days i wait for them first to avoid disappointment.
The good ones nod. The ones that don't I generally just regard as dick heads. I'd still stop for them if they were at the side of the road.. old habits