I think it just comes down to what the plan is for the ride. I think for most roadies, the plan is to go explore and get a workout, maybe stop for a quick coffee, but mostly it's on the go. So most food is in a form that easily stuffs in a jersey pocket or dissolves in a bottle of water.The rides can tend to be longer and faster, and a backpack really makes one feel slower and less nimble on the bike.

Back in the day I used to do trail runs - 10 to 12 miles with nothing but a bottle of water in each hand and a hip belt with my phone and a few calories. Then when friends would plan a "hike" they always wondered why I wasn't carrying water or a backpack for a 5 mile stroll on a trail. More recently I've been getting into hiking more with my wife, as kind of a way to slow down and not be super intense/competitive all the time. We always carry a backpack now because it's really pleasant to have a sandwich, a couple beers, and perhaps a joint at some gorgeous secluded creek or mountaintop. The trail run was to be fast, a workout, get in and out kind of thing. The long chill hikes are an all day, hang out, sometimes stop and relax kind of thing.

It's a similar deal on the bike. Often times I kit up with nothing but gels, water, and my usual pocket stuff for a "workout" or "quick" ride. "Quick" doesn't necessarily mean only a couple hours - it could be a 4+ hour ride, but still the point of these rides is to ride, mostly nonstop, often never even clipping out of my pedals door to door. Other days I meet up with friends, explore some random trails, climb hills and chill out for a bit, go to the beach and hang out while the sun sets, etc. The point is more relaxing, hanging out, less about the bike itself and more about the rest of the experience. I'll bring a backpack with a spare jacket, a few beers, snacks, etc. I'll wear street clothes. It's like going to hang out when you're a kid and you ride bikes to get to the places you're hanging out but also just ride around.