I cycle for adventure and experience, it's also why I prefer gravel and offroad cycling to road. That gravel crunch in the forest, smell of the trees, sound of birds and absolutely no traffic is pure bliss. Ok sure there is some suffering during climbs, but the feeling of accomplishment when you finally get on top and the descent that follow are worth it.
I join one gravel competition a year, but I go for the event, not to actually compete. I like riding with one friend, on paths where we can ride side by side and chat, and I don't see a point in group road rides where you just stare at the butt of the rider in front of you the entire ride.
In general I experience joy the entire bike ride, it's like my therapy, a couple of hours without a phone, outside, just me and silence. I guess the fitness that comes with it is good too.
Yeah I can see a treadmill being extremely boring. I usually ride and run outside, but I had to ride on a trainer for a couple of weeks because of an injury and it was just awful. I couldn't wait to be done. But when I'm outside sometimes I'll extend my ride for another 10km just because the weather is nice and I feel good, I'm (almost) never in a rush to finish.
I don't think I could run on a treadmill either, at least outside I have short goals to reach (ok now to the next lamp post...)