I think it’s the tendency to be heavier on the throttle. I dirve my Suzuki car like a granny; I drive my Suzuki motorcycle like a banshee.
I hear you though, and I’ve wondered the same thing many times. Many bikes are still carbeurated; would that be a factor? Compression ratios? Bikes are way higher compression engines than cars. It must take more gas to create that kind of combustions.
I’m just throwing ideas out there; I really don’t know.