"I'm not actually sure how much magic house elves can do besides Apparition. But their masters do feed them and shelter them. They could maybe survive in the wild - or at least they could if they wouldn't be really depressed about it all the time - but they might have some deficiency in, say, figuring out what to do next after doing a thing, or forming societies big and interconnected enough to prevent inbreeding, or something like that, and they solved those problems by getting humans to fill in. I'm not sure about any of this, you understand, I'm only guessing."