Hey there! this game is trying to be more or less realistic (in its handling of relations between sovereign at least). That said, where have you seen that in life one could not combine with over 80/100/150 people? These alliances self-regulate: if you want more freedom and if you think you can progress without many allies and with more potential targets, you reduce your number of allies by changing tribe (or kicking players if you have your own tribe). This is life. Then this is why you will never see top10 in the same tribe : there is no challenge and you have many more to get when you are the first player if you don't ally with the 2nd or the 3rd.
(I hope i'm understable; beeing french it is hard from me to speak english well).