Because the ethics committee is never going to sign off on an experiment that has even a hint of risk to mother or child who aren't already at greater risk from

Because the ethics committee is never going to sign off on an experiment that has even a hint of risk to mother or child who aren't already at greater risk from a lack of intervention. It's the same reason you don't conduct Phase I clinical trials of new chemotherapeutic agents with people who don't already have cancer. Birth is already risky enough. "Let's science that, to see which has the best outcome," while a completely rational approach not only to decreasing harm, but also to increasing health, just isn't going to fly.