Because there can be only one authority for ".".
I don't agree with this.
This must be because you've read RFC1034 and RFC1035 more recently than I have, and have implemented your own DNS servers which works better than BIND, and you know something about DNS architecture that continues to elude me?
It is quite possible for everyone to maintain ".". Then, there would be no need for root domain servers; HOWEVER, this would take massive cooperation throughout the industy, which will never happen.
I said "one authority". That means "one definition of its content". Sure, if everybody in the industry could agree on all changes, even minor little "add an NS" or "change an NS" changes, then they could all run "." as primary masters. But there would still be only one authority. Since we know democracy doesn't work at that level of granularity, we usually elect representatives. The current nonelected representative is Jon Postel, who is trying very hard to hold free elections without destabilizing what passes for government at the moment.