installs all those things and sets things up properly on a standard fedora install?

That's exactly what all universal blue images do. It's just that setup is done every single day in github from scratch and stamped out as an image so that the end result gets to your computer as a finished deployment artifact. Leads to better update reliability, built in rollback.

The biggest benefit is that it's easier for a community to fix the fast moving gamer stuff as a config layer on top of a distro that's delivered this way than me having to manually figure out what component of my gaming setup changed that week.

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

You could do that. With that image everything is vompletely equal on the user device which means that debugging is much easier. Ublue makes distributing custom fedoras increadibly easy.

How does it have a large attack surface? I thought being immutable reduced the surface.