Wearing a knapsack is the last thing i want to do riding a bike
I used to say the same thing. Then on one trip in the desert southwest I wore a camelback full of ice and found it comfortable enough. I figured that was mostly just because I was enjoying the wet and cold of the melting ice. Then a few years later I wore a little backpack that I carry for going to the store or toting extra food and water when necessary for a longer stretch and found that when it wasn't needed for the extra capacity I still wanted to wear it. So I guess it wasn't just the ice, the backpack wasn't bad after all.
Since then I have been enjoying using a little UL backpack with not much stuff in it. It is nice to have a few items automatically go with me when I get off the bike to go into a store or diner or go for a short hike or whatever. I never put more than a few pounds in it, typically no more than 2 or 3 unless there is the need for carrying water for a long stretch between restock points. While it is only 2 or 3 pounds It is a significant portion of my load when packing ultralight. Also it is a great way to have the things I expect to need throughout the day easily at hand.
I find that I like to use something like either the 2.5 ounce Sea to Summit Ultra-Sil pack or my old REI Flash 18 pack at a bit more robust 9 ounces. I really like them both and which one I take depends on the usage expected. In either case I don't find it all that much different than carrying a bunch of stuff in jersey pockets and when I expect to wear the backpacks I am likely to forgo the jerseys in favor of a tech tee.