People with many types of physical disabilities need a wheelchair specially fitted to them. They may need a headrest, or a certain type of harness, or the right seat depth. This is going to be particularly true for a child who is likely too small to sit in a airline wheelchair.
An airline wheelchair, or a Disney one for that matter, is most appropriate for people who can walk a little, but not far enough for an airport or a theme park.
In addition, a wheelchair or a specialized stroller is a very expensive piece of equipment, that requires a physical therapist or other specialist to measure for and select. Someone's trip, and the few months afterwards, would be ruined by a missing special needs stroller, in a way that a parent who lost a regular baby stroller wouldn't have to deal with.
Better to let people use their own in this circumstance.