[1] https://help.apple.com/pdf/security/en_US/apple-platform-sec... [PDF]
Android handles it the same way.
Average Joe has to grow up. Scam websites and scam callers aren't going away, and even scam apps are being approved and distributed on Apple's App Store. On iOS, you have to do your due diligence to avoid being exploited (and even that might not help against some threats).
You’re talking about a user who has gone out of their way through multiple scary Apple warnings to change their browser engine.
And even once they do that, they’re likely to be installing one of Firefox, Chrome, or Edge, all of which have as good if not better security histories than Safari.
For sure Apple doesn't like the macos security model and would want way tighter control on what's allowed to run, if it could get away with it. But it's not about what Apple wants, it's about what they're asked to do.