That would be very very hard and unreliable.

Bazzite is more than just "preinstalled Steam", it has a list of tweaks, optimizations and additions so long you can't even finish reading it all! 😅
This includes a different kernel, pre-configured containers, and much more.
If you do that on a regular system, configuration drift would quickly destroy any good experience in no time and result in a huge mess.

uBlue provides a solid base distribution (pretty much stock Fedora) and applies exactly your way, but in upstream, and then copies that new image to millions of PCs. By doing that, you can provide many many identical copies that are the same everywhere and always up to date, without the burden of maintaining a whole distro like on Nobara.
The hard and boring work of maintaining a distro is on the shoulders of the Fedora team, and you only have to maintain your own changes.

This seems something with too big of an attack surface.

Not really.

• Most stuff is installed in containers
• The pros of image based distros still apply here in terms of reliability, security, etc.
• Its no more than a few hours away from upstream stock Fedora
• Most apps (Lutris, OBS, etc.) are optional and opt-in, if you just click "next, next, next" in the installer you'll get a relatively vanilla experience compared to stock Fedora