Back when I was a runner, I never did it because I enjoyed it; it was a means to getting fitter or because I was training for something specific (usually a much longer run I'd planned in the future). Because of that, when I was out I was focused on just getting it done (and getting it over with as soon as possible!) with my head down and would rarely notice other people except to avoid them as though they were just obstacles I didn't want to run in to - mobile lamp-posts if you will. Plus I rarely had much breath left that could be used to say "hello" even if I'd wanted to.
Whereas when I'm out on my bike, I'm there because I want to be - I'm enjoying myself and the fitness/excerise I'm getting is just a happy by-product. So normally when I'm riding, unless I'm doing something silly like sprinting for a village sign or coming to the top of a big hill, I'm usually in a comfort zone and able to react to my fellow riders positively because I'm in a good mood. Or at least that's my excuse.