There’s an obvious difference between the two: having a baby is not the same as giving away an organ.

Humans have a finite number of organs which they need to live. You are selling a finite resource and doing so has concrete drawbacks for the seller.

Having a baby does not give up anything. It is a win-win. Surrogates in the US are asked if they are done personally having children. The agency won’t let someone be a surrogate (usually) unless they affirm they are done having children of their own.

As an aside: a highly regulated market for organs would probably be a net positive for society. It would increase the total supply of organs and decrease demand for black market organ harvesting. It would almost certainly save many lives every year.