If someone can lift 100 pounds, would you expect them to carry it with them everywhere they go?
If someone can sprint 20 miles per hour, would you expect them to be able to maintain that speed for a long distance?
Of course you wouldn’t. Being able to do something does not mean that you can do it for a long period of time.
So if a wheelchair user can stand to reach something or walk a short distance, why do you assume they don’t need the wheelchair?