I ride with a backpack almost every time I get on my bike (I use my bike almost exclusively for commuting), but with that said I'm not going to disagree with you because I've never worn a hiking pack while riding
The only thing I can think of is; wouldn't a hiking pack interfere with your helmet when you're leaned over in riding position? Unless your riding position is extremely upright. I would think that you wouldn't want the pack to rise over your shoulders (aerodynamics & potential helmet interference), and you also wouldn't want it too low if you've got a solo seat cowl on ur bike.
My pack is just a regular "swiss army" $40 bag, and I keep a small plastic bag in it which I can fill with anything sensitive to water, should I anticipate a wet ride.
The majority of their packs only go up that high if you strap something in the top of it or get the Long version when you should have something shorter. Most of their packs are designed to stop at your shoulders. Maybe it would interfere on an SS but I doubt it unless you've rolled a sleeping pad and stuffed it into the top or are wearing it way up high with a Long pack. On my 650R with sportbars it's not a problem.