If you're talking about sidewalks with separated pedestrian and cyclist traffic, some bike lanes are effectively that (e.g. raised to sidewalk level, left turn boxes on the right instead of forcing bikes to cut across traffic). They don't seem to be very common. Perhaps it's because it is a newer idea that people are unfamiliar with or maybe it is because it is less expensive to take some space from oversized lanes than it is to widen sidewalks.