I don't even get why I have to "sign up." I'm already a paying customer with an existing account.

Billion is table stakes , OpenAI has raised over 6 billion dollars this year alone

This is by design, they want the news articles saying "this is so popular it crashed their website!"

That’s because in this case scaling to big traffic needs more hardware which is very expensive and even if you have money the manufacturers may not have the capacity you need.