If you have a wide enough definition of AGI having a baby is making “AGI.” It’s a human made, generally intelligent thing. What people mean by the “A” though is we have some kind of inorganic machine realize the traits of “intelligence” in the medium of a computer.
The first leg of the argument would be that we aren’t really sure what general intelligence is or if it’s a natural category. It’s sort of like “betterness.” There’s no general thing called “betterness” that just makes you better at everything. To get better at different tasks usually requires different things.
I would be willing to concede to the AGI crowd that there could be something behind g that we could call intelligence. There’s a deeper problem though that the first one hints at.
For AGI to be possible, whatever trait or traits make up “intelligence” need to have multiple realizablity. They need to be at least realizable in both the medium of a human being and at least some machine architectures. In programmer terms, the traits that make up intelligence could be tightly coupled to the hardware implementation. There are good reasons to think this is likely.
Programmers and engineers like myself love modular systems that are loosely coupled and cleanly abstracted. Biology doesn’t work this way — things at the molecular level can have very specific effects on the macro scale and vice versa. There’s little in the way of clean separation of layers. Who is to say that some of the specific ways we work at a cellular level aren’t critical to being generally intelligent? That’s an “ugly” idea but lots of things in nature are ugly. Is it a coincidence too that humans are well adapted to getting around physically, can live in many different environments, etc.? There’s also stuff from the higher level — does living physically and socially in a community of other creatures play a key role in our intelligence? Given how human beings who grow up absent those factors are developmentally disabled in many ways it would seem so. It could be there’s a combination of factors here, where very specific micro and macro aspects of being a biological human turn out to contribute and you need the perfect storm of these aspects to get a generally intelligent creature. Some of these aspects could be realizable and computers, but others might not be, at least in a computationally tractable way.
It’s certainly ugly and goes against how we like things to work for intelligence to require a big jumbly mess of stuff, but nature is messy. Given the only known case of generally intelligent life is humans, the jury is still out that you can do it any other way.
Another commenter mentioned horses and cars. We could build cars that are faster than horses, but speed is something that is shared by all physical bodies and is therefore eminently multiply realizable. But even here, there are advantages to horses that cars don’t have, and which are tied up with very specific aspects of being a horse. Horses generally can go over a wider range of terrain than cars. This is intrinsically tied to them having long legs and four hooves instead of rubber wheels. They’re only able to have such long legs because of their hooves too because the hooves are required to help them pump blood when they run, and that means that in order for them to pump their blood successfully they NEED to run fast on a regular basis. there’s a deep web of influence both on a part-to-part, and the whole macro-level behaviors of horses. Having this more versatile design also has intrinsic engineering trade-offs. A horse isn’t ever going to be as fast as a gas powered four-wheeled vehicle on flat ground but you definitely can’t build a car that can do everything a horse can do with none of the drawbacks. Even if you built a vehicle that did everything a horse can do, but was faster, I would bet you it would be way more expensive and consume much more energy than a horse. There’s no such thing as a free lunch in engineering. You could also build a perfect replica of a horse at a molecular level and claim you have your artificial general horse.
Similarly, human beings are good at a lot of different things besides just being smart. But maybe you need to be good at seeing, walking, climbing, acquiring sustenance, etc. In order to be generally intelligent in a way that’s actually useful. I also suspect our sense of the beautiful, the artistic is deeply linked with our wider ability to be intelligent.
Finally it’s an open philosophical question whether human consciousness is explainable in material terms at all. If you are a naturalist, you are methodologically committed to this being the case — but that’s not the same thing as having definitive evidence that it is so. That’s an open research project.