Preventing abuse? It's much harder to create a throwaway phone number than a throwaway email address.
> OpenAI does the logical thing. Let's me enter my credit card and I'm good to go. I will stay with them.
You'd rather hand over your credit card than your phone number? I think most people would see it the other way around.
You know, that was my first reaction, too. But really, my phone number is much more integral to my identity. I can cancel or change a credit card number pretty trivially and then it's useless to you.