Because it's far more cruel to pretend to care for someone and then drop them the moment it no longer suits you, than to help them find the care they want in their other relationships.

Your therapist isn't supposed to be a substitute mother/lover/whatever they're someone who helps you facilitate healthier relationships in general.

Also, I don't believe for a second that the illusion is all most people will have. But the belief that it is is certainly a major barrier to finding it.