Dizzying rent-seeking fees
This ensures that none of the "browsers" can compete on iOS and this obvious by comparing browser market share of the same browser between iOS and Android.
As you say, Apple only allows their own WebView to exist on iOS, which is an engine they both control entirely and is heavily locked down. Not helping matters is that WebView runs on WebKit (Safari uses WebKit as well), which is these days pretty much the equivalent of Internet Explorer in terms of browser shenanigans[0].
The result is that the only real thing you get from Firefox/Chrome/Edge on iOS is access to your synchronized bookmarks. Apple doesn't offer any form of a WebExtension implementation either to these engines (instead rolling their own version, which they confusingly also call WebExtension), and none of the previously mentioned browsers are even allowed to add the universal form of WebExtension support to WebKit. The result is that iOS also remains one of the few platforms where meaningful adblocking remains a crapshoot (entirely beneficial to Apple of course).
[0]: To be somewhat fair here, WebKit *is* very useful for more embedded/low powered devices that aren't intended to access a lot of websites to begin with. There are some uses for WebKit, IE had none near the end.