Mostly because it can't be enforced effectively enough to warrant doing it in the first place. And what is government other than some random people? As you said, people who want kids will have them regardless. I don't think that's a good thing. Children are people too, not just something an adult gets to have because they want one. you're stopping short of saying the standard should be enforced to avoid the whole government thing.
So what about normalizing adoption ? Currently it's seen as the last ditch effort to have kids for those people who fail in every other way. If it wasn't viewed so negatively than maybe more people would make that choice when deciding if they want kids. There are actual ways to convince more people to stop having kids they know will be sick that don't involve any sort of voting or standardized list of who can acceptably have children.
Dude, it's not about looking down on people or moral failing. The more you look down on people, the easier it is to dehumanize them and the less you will care about their suffering. That's not difficult to sell, it's just an approach that will take a new generation to fully understand. Considering adopting would have less of an impact on a woman's career since she skips pregnancy it could easily start to seem like a positive.