Of course different manufacturers produce flagship phones, its not a problem.
iPhone has been present since 2007, Android hasn't started growing until late 2010/early 2011. Even with the iPhone having a headstart, Android still leads in marketshare,over 50% as compared to 30% on iOS. Rudimentary statistics knowledge is all that is necessary to understand that having one device with a large marketshare is hard to sustain for long term success.
https://forums.macrumors.com/threads/1353165/
Android OS offers choice, iOS doesn't which is why I feel bad that everyone is forced to buy one edition of the iPhone if they want the latest one.