I was just about to observe that starting immediately was a thought-through decision, based on the awkwardness of waiting for a whole bunch of people to click accept.
That said, I will confess that starting immediately was also by far the simplest thing to do, and it is not set in stone.
Maybe for live games, an accept by each player makes best sense, and looking at what we have now I can see that it may be able to be done reasonably elegantly, so I will put that on the list.