Because they didn't make any effort to treat this like a trade mark until after the term itself become generic to the public. If you want a trademark you need to be careful how you use it and how others use it to ensure it doesn't become a generic term. Once something is a generic term it is almost impossible to get it back. (it has been done: Xerox used to be the generic term people used for make a copy - but most of the effort was their competitors who for obvious reasons didn't want to use their competitors company name as a generic term)
If you want a trademark you need to defend it. That means you know the generic term and use that when required. That means when anyone uses your trademark in a generic way your lawyers are immediately sending letters. Check with a lawyer - there are a lot more details you have to get right.
Yeah, just because OpenAI has gotten away with using open source ideology for regulatory capture in the AI space, but that doesn't mean they can get away with it in the intellectual property space.
