The problem with having a baby for someone else is that doing so is predicated on the assumption that the pregnancy will go well. One of my cousins had preeclampsia and early rupture of membranes. She had to spend weeks as a hospital in-patient, having her blood pressure monitored 12 times per day.
Because the baby was hers, the decisions as to when to induce, how often to monitor, whether to nip out to Mamas and Papas between blood pressure checks, etc were between her and the medical team because it was her baby and her body and she owned all the proxy decision making for the baby morally and legally with no social pressure from a commissioning egg provider. It was up to her how much risk she was prepared to take when balancing the benefits to her baby against the risks to herself, and it was her risk to take with her own body to benefit her own baby.
If she had been a surrogate mother, the commissioning egg provider would have had opinions and, no matter what the law said, there would have been social pressure from the egg provider for my cousin to put her body on the line. There'd also be some people who would try to make a moral argument that the egg donor should have a right to interfere in the surrogate mother's medical decisions, dictating or influencing the level of risk, inconvenience, and suffering a surrogate mother should endure to save what is genetically not her baby. And it's easy for the egg donor to insist on that because the egg donor isn't the one taking the physical risk.
Altruistic surrogacy lends itself to destroying relationships between family members if it all goes wrong. The picture in my head is of one of those "women's gossip" magazines with "My sister ABORTED my baby" on the front cover because the sister couldn't, or didn't want to, endure the pregnancy complications.