iPhones aren't allowed to have web browsers that aren't running Safari on the back end. They can have different names, different UIs, but at the end of the day it's the same browser.
I don't know why non iOS browser play the stupid pseudo support game with Apple, it just makes it confusing for users like the parent comment... playing into Apple's hand, helping them pretend that it's actually a fair ecosystem with choices.
That's easy. Users want their bookmarks, history, passwords, etc synced across devices, and so if there's no Chrome, Firefox, Edge etc for iOS to accomplish that on iPhone/iPad, people who have multiple Apple devices would likely switch to Safari and those users would be lost. Additionally, in the case of Google and Microsoft, they'd be opting out of those sweet, sweet streams of user data if they had no iOS presence. And so, those browsers exist, just built around WebKit instead of Blink/Gecko.
Apple does this for a few reasons:
1) to prevent web apps from gaining a foothold. Modern web standards aim to provide native-like utility, performance, look, and feel. Apple purposefully holds their device back so companies are compelled to deploy native apps.
2) to prevent Google from taking over their platform. Chrome could give Safari a run for its money and "steal" user attention away from Apple's key platform pieces, providing a foothold for Google on iOS.
i have an iphone with safari, firefox, firefox focus and chrome, ...