The reason, is that bandwidth is a solid profit center for the cloud majors. Amazon initiated it, the rest are going along in cartel fashion, nobody wants to disrupt one of the lucrative angles by driving that profit center toward zero.
There's no other rational explanation why Azure or Google aren't hammering AWS on bandwidth cost as a competitive angle. They're basically conspiring in the industrial sense, no different than airlines that somehow magically agree to simultaneously raise prices or department stores that for decades all agreed not to discount the perfume etc. box section of their stores.
