Some racers do acknowledge, some don't, depends on the person I suppose!
What I have noticed, however, is that there are some racers who appear to a little too obsessed with the whole shebang.
This evening was a case in point! I do a 20-mile route of an evening sometimes, particularly when I've been sat at the PC all day. I am pretty quick and I have an ultra-skinny weight-weeny bike, but I do like to take in the scenery!
I stopped mid-way round to take in the sunset over the sea. It was the kind of picture-postcard scene that makes you glad to be alive. Truly stunning. Then a group of three roadies pulled up beside me, one glanced at me and said, "Hi" (I responded, of course!), then they all got together in a little circle, heads down, comparing various gadgets that had flashing lights and made bleeping sounds, one of them excitedly said, "Fifteen point two! Yes!", then they got back on their bikes and blasted off (heads down, looking only at the road).
At no point did they look up at the sunset for one second. They didn't even notice it!
While they may well have been in training (for something, dunno what, they weren't in team colours), I think they were typical of the kind of racers who I would describe as a little "soul-less" - everything's performance, performance, performance - it's as if they've "lost touch" with the pleasure of cycling.