[barrkel]

You expend labour; they build capital. The capital you build produces returns long into the future, but you don't own any of that future income stream. Sooner or later, your sharpness and skills will decline, and your labour will be relatively worthless. The capital you helped build will continue to be valuable (presuming you did a good job).

It's a poor deal if you're in a competitive market where there's another schmuck willing to do your job for just as cheap as you do it. But if you have pricing power, you'd be a complete fool not to either charge substantially more, or demand some ownership.