Probably because it's a recognizable name, and the overwhelming majority of users don't seem to notice or care.

So... business purposes. I love FOSS and see the irony just as clearly as anyone else, but it's kinda pointless to highlight supposed hypocrisy from the name alone. It would be stupid to get mad at Alphabet for not inventing new letters or Apple for not breeding new fruits. Yes, the name implies something that it's not. Outside of the fairly limited (and clearly unrelated) context of Open Source though, it doesn't seem like their name correlates to much of anything.

Long-term, it wouldn't surprise me if they go all-in on the ChatGPT name and switch OpenAI to something more suit-and-tie. I'm sure they're in no hurry, though.