Because there needs to be a payable date to decid

Because there needs to be a payable date to decide who is going to get the shares. Even though there are no publicly traded shares yet, it will be a book entry item on the books that you are entitled to the shares when they go public.

Disclaimer: Of course, all of this is my opinion and you should not make any investment decisions based on my opinion. I have not received any non-public information.