Let me answer that by picking the question apart:

1. To update the system, you need access to write actual system-files. You need system-level access.

2. Android is built entirely up on a "user-space" model where permissions are granted to apps, and almost no apps have system-level access.

3. On rooted phones you can allow system-level access to specific apps, which in theory could install the latest OS version and binaries. In theory that sounds like 1 app which you can use to update the OS, everywhere, right?

4. Except you would need a different installation-strategy for unrooted phones anyway for the installation anyway.

5. To compound the problem the ARM platform has no standardized BIOS/UEFI layer to handle generic booting, so the kernel being booted must be device-specific in order to gain access to the rest of the phone: It can't rely on generic "BIOS"-drivers to load modules on demand from flash storage. And the boot-partition is almost always too small to have a kernel with enough drivers for "everyone" embedded.

6. So you cant make a standard phone-agnostic flashing-app, nor install medium. You do have to build a custom phone-specific initial flash-package for every model you want to support as a bare minimum.

And when you've already put in all that effort, what do you gain by creating an "app" which does this inside a pretty GUI, instead of just rebooting back to recovery to (auto) flash the latest image? Practically nothing.

"Flashing" may sound scary, but in reality it just means installing. There's no technical difference between that and just copying files to a usually write-protected partition.

AFAIK: Apple officially does exactly the same, they just don't call it by that name.