Why is staying at home bad when disabled?

Isolation. This can lead to depression and social isolation. Get out, get active and get involved. Disabled doesn't mean you have no purpose, no place and no function. You can do it.
Isolation. We are social beings. We are meant to interact with others. When we lack social contact with others we have a tendency to become withdrawn and depressed. This negatively affects our health. There are certainly some barriers to getting out of the house when disabled but they are not insurmountable in most cases.