Why is it that to many people, "disabled" means "wheelchair users" and
absolutely nothing else?

"Disabled" covers a wide range of physical problems, many of which do not
involve people needing wheelchairs.

As an example, just look at the number of people on public transport who
need a walking stick to get around.

Deafness, blindness . . . . . the list is endless, but they are all serious
problems for the individuals concerned, and yet so many people just think of
wheelchairs.