In low density areas that don't see a huge amount of either traffic, there's no reason not to mix them. It works just fine in plenty of places.

mixing bicycles and pedestrians is perfectly fine, you only get problems when people don't have anywhere to bike quickly.

Most cyclists are happy to go 15km/h or slower, which works flawlessly with pedestrians so long as volumes of either aren't huge. But there's always some percentage of cyclists who want/need to go 20+km/h, which is only acceptable if there's barely any traffic at all.

Frankly it can be really nice to mix slow cyclists and pedestrians, because it means more opportunity for people to talk to each other. I have several times met people i know while on my bike, and because my city at most separates cyclists from pedestrians with paint it means i can just stop and have a chat with them, it just feels nice.