And you can only hop on/hop off a cable car at predetermined locations. To keep average speed high, those tend to be spaced relatively far apart.

I’m not sure about the “takes a lot more space”, and I definitely doubt about the “costs a lot more money”. Using outdoor escalators as proxies, I suspect outdoor moving sidewalks will need lots and lots of maintenance. If you want to have some guarantee of service you’ll also need multiple sidewalks side-by-side.