How Apple distributes their core apps (Safari, Mail, etc) is orthogonal to how they are implemented, secured, ring levels, sandboxing, etc. These are separate considerations.
Apple's core philosophy is that upgrading the system is the fundamental way to get new things. With iOS 15 we've seen the first real fracture in this model (where they are promoting a "stay on iOS 14 for now" option), and maybe eventually they'll separately distribute some of the tied applications.
Google started Android with a very similar model to iOS but quickly recognized it was turning into a disaster given the slow uptake of new Android versions. Turning what were system level components (e.g. play services) into "apps" was a necessity.