But if the web engine you have to use is a) shitty and b) the same as everyone else (except slightly shittier [1]), how are you going to make it popular?
The browser engine is an important piece of the puzzle when it comes to getting people to switch browsers. If a site works faster or better on browser B, that's a reason to switch.
If Apple's WebKit engine doesn't allow you to do video calls, you could build that into your engine and actually compete.
Why would Google and Mozilla spend millions on developing and maintaining their own engines if they didn't think that it provides value, e.g. a competitive edge?
[1] In the past, Safari did get some features that other browsers on iOS didn't get, e.g. Being the default browser (you couldn't set Chrome or any other browser as the default until iOS 14(!), Nitro JIT (Safari was significantly faster than other browsers on iOS for 4 yrs), camera and microphone access (video calls only worked in Safari), Web Extensions, Apple Pay, Fullscreen API, Add to Home Screen (ironically)...
