But they are? It's a rendering engine monoculture. Sure, they might have different skins and some stuff bolted on top, but let's not pretend that that constitutes a different browser (and this is precisely why Apple got bitch-slapped by the European Union).
It’s still something of an exaggeration. If you take a look at the source for iOS browsers, the amount of unique code is non-trivial.
At minimum, it’s a sliding scale rather than binary and iOS browsers are less Safari reskins than Chromium-based browsers (most of which share a much higher percentage of code) are Chrome reskins. There’s exceptions like Arc which uses a bespoke AppKit/SwiftUI/WinUI UI instead of the standard Chromium stuff but that’s pretty rare.
Orion has proven that web extensions are allowed, even if its implementation isn’t complete. There’s no guideline preventing a browser with a user-hackable UI (like Firefox userChrome) but nobody’s tried that yet. What’s left? As far as I’m aware, it’s just the small handful of manifest v2 request interceptor APIs that uBlock Origin depends on that can’t be supported fully.
But they are? It's a rendering engine monoculture. Sure, they might have different skins and some stuff bolted on top, but let's not pretend that that constitutes a different browser (and this is precisely why Apple got bitch-slapped by the European Union).