First, "outcome of the kids" is a subjective standard, so I'm not really sure what that means.  Second, our legal standard in the US is the BEST interest of the children, which many years of history has shown us is the mother-father-child household; and while a surrogate child might be "doing well," how well might that child have done with his/her own two parents raising him/her?  Third, is that the outcome of our children is a matter no just of the child, but of the future culture and society in which that child lives.  Thus, if we promote, e.g., single parent creation of surrogate children, what standard are we setting for society, and what is the good/harm thereof?

Again, some single parents raise great kids, but those situations also serve as a model for poor child-moms who think, "Hey, that woman has a child, so why shouldn't I?"  And, of course, the outcomes of those children are statistically on a track of the poverty that the mom has.