We are consummate individualists.  But we consider people socially independent when the social interaction they (like everybody) depend upon comes on account of their merit.  Someone without a disability holds down a job and the social interaction from said job (and the resulting relationships formed through it) help sustain them socially.  The point is people are coming into this person’s life through sheer self-interest.  Now someone with a disability who can’t get a job due to prejudice and other barriers doesn’t get this social interaction.  So they’re effectively a shut-in.  Engaging them is considered an act of altruism and done generally only by helping professionals.  This person isn’t considered independent because it isn’t in others’ best interest to engage him socially.  The point is “social independence” is a construct of society based on society’s opinion of you, nothing else.