I simply don't understand why roundabouts aren't just shared. Do separation as much as you like 10m away from them, but in the roundabout, there's exactly zero

I simply don't understand why roundabouts aren't just shared. Do separation as much as you like 10m away from them, but in the roundabout, there's exactly zero excuse for a speed difference between motor vehicles and bikes. When drivers are expected to let segregated cyclists pass on leaving the roundabout, they actually have to go slower than they would merged behind even the laziest cyclist. I really don't get it. (yes, the answer will be "but children!", but I'm a believer in "treat cyclists like children and only children will ride bikes!" and I'd be a firm believer if the Netherlands didn't have this annoying habit of proving me wrong ;))