Visiting other people's homes is as important to people with mobility limitations as it is to other people. And, finding the rare accessible house or apartment to rent or buy at the time it's needed is often impossible. The two most important needs for visitability are getting into the home and fitting through the interior doors, especially the bathroom.
Access looks unattractive.
Access is expensive. In renovation, depending on the situation, adding basic access to houses or apartments may or may not be expensive. A zero-step entrance is feasible only on a flat lot. A zero-step entrance is only feasible when building on a concrete slab. Building with a basement or crawl space does not deter a zero-step entrance.