

I mean… they sometimes are (if the sidewalk is designed for it), look at multi-use trails. A city near me allows bikes (coming from the trail) on wide sidewalks to the main street.
It depends on the flow of pedestrians (too many people would be difficult to navigate with a bicycle anyway) and it can be a visibility issue with doors of storefronts (especially as people leaving likely aren’t expecting/looking-for someone passing on a bike).
A lot of issues like this are how things are designed. Taking a page from NotJustBikes (look them up if you haven’t heard of them), lots of things are car-centric (cities, housing, zoning, parking-lots, lack of public transportation) even when it comes as a detriment to everyone not in a car (and sometimes even those in large vehicles, because congestion).
It’s also another culture-war thing and not even just in the US, look how in Canada Doug Ford wants to remove even the painted bike lane.