Because people cannot jump over or walk up high walls, society developed stairs, elevators, and escalators. They can't jump from floor to floor. Steps were designed by architects to enable people to walk easily from floor to floor. Ramps for people using wheelchairs are not a special accommodations for a deficit in the individual's ability to climb stairs, but are viewed as corrections to deficits in the environment, no different from steps or doorways. Escalators were made as a convenience for some people.
Elevators are a convenience for people as well.
That one person finds elevators convenient for his or her wheelchair is viewed as an environmental, not a personal issue. The Independence Model labels older thinking as the Deficit Model. The Deficit Model finds the problem to be in the individual and seeks to correct the problem by changing the individual. Closely allied to the Deficit Model is the Medical Model. The focus is on fixing the individual.