It goes back to 1886 [1]. Ditching corporate personhood just makes the law convoluted for no gain. (Oh, you forgot to say corporations in your murder or fraud statute? Oh no!)
It gives rights, not obligations, which makes Citizens United so abhorrent. It's a dark money vehicle, and worse - foreign dark money. Just on the face of it its ridiculous, but alas, laws are made by the ultra rich.
A corporation has the right to "speech" but if crimes are committed, rest assured it will not go to jail, and neither will its executives, protected by layers of legal indirection of this "person corporation".
Musk didn't pay for everything. He took his money and left, upset that OpenAI wouldn't let him run it. It was precisely because Musk stopped funding them that OpenAI were forced to seek outside investors and change their corporate structure to be able to offer them a return on the investment.
Obviously this was originally presented as a non-profit, so not a normal startup by any means, but certainly it is normal for startups to "pivot" direction early on and end up doing something completely different than what they initially said. I'm not sure at what point this might upset investors, but I believe the idea is that they are usually investing in the team as much as the idea.
The idea of corporations as legal persons predates the United States. English law recognised trade guilds and religious orders as legal persons as early as the 14th century. There is nothing specifically American about the idea at all-the US inherited it from English law, as did all other common law countries-and English law didn’t invent it either, similar concepts existed in mediaeval Catholic canon law (religious orders as legal persons) and even in Ancient Roman law (which granted legal personhood to pre-Christian priestly colleges)
