I don't always say hi, sometimes I don't even acknowledge the other rider's existence. Some days I might be feeling super social and ride with a stranger for a few miles and pass the time of day.
Some riders are new, they're basking in the golden glow of a new love discovered (cycling). Some of us have been doing this for 30 years or much more, we're not in the honeymoon phase anymore, but we love cycling and that's why after all these years we're still doing it. To some of us, cycling isn't unusual, it isn't even an activity, no more than walking to the shops is an activity anyway. So it doesn't seem unusual to see someone else on a bike, that's why we sometimes don't acknowledge you. There are other reasons we sometimes don't acknowledge, some days we might be on the tail end of a hilly 150 mile ride in the wet (it might have been raining 75 miles away that you never saw), we're on our way home tired, bedraggled and dreaming of cake, waving hello is not on the top of our agenda.
I'd say that for the most part, you'll get a nod or a wink from me, might even raise my fingers from the bars with a half hearted wave.
Sorry.