Well, if there is an RCE vulnerability, that would allow the user to run arbitrary code on their own device with the same permissions that the browser process has, which is clearly unacceptable in the iOS world.
I don't see how that could be acceptable in macOS world.
On macOS you as the user already have complete and unrestricted access to your computer. You don't jailbreak your mac because it already comes with root and an unlocked bootloader. Mac apps also have much more access to the underlying system than iOS apps do, and this helps with better sandboxing, I presume.