Sure. They could do that. That doesn't really address my point though. And it's really unlikely to happen on any meaningful scale, imo:

1. The market for a dedicated phone OS from one of the larger consumer electronics makers is ORDERS of magnitude larger than any kind of handheld PC gaming console. Just because Samsung did it for phones does not mean anyone will do it for handheld PCs.
2. Even if they do, there's a lot of negative sentiment about Samsung's version of Android, to the point multiple Android users I know will never buy a Samsung phone. It's not necessarily a goal to emulate.
3. Leaving all of that aside, that is still not the same thing as the maker of the device also being the developer of the OS. You're at the whims of upstream to fix a lot of major things, or you're maintaining a massive patch process on top of their releases. It's a much larger task than just "hire staff to optimize steamOS".

We already have some makers offering "steamos support" in the form of... basically a single steamos image they release once and don't steam to maintain? GPD's "GPD OS" from Dec 2022 and Anbernic's Win600 Steam OS image from Jan 2022.

And still the best way to run SteamOS on either of these devices is ChimeraOS.

The closest to what you're describing is AYANEO's ayaos. I don't know if it's a steamos fork or not, but it's their take on linux gaming OS. It's been in development for a while and we've got nothing but a few clips of it to view. And considering it mostly seems to replicate the Ayaspace windows app interface, I'm not sure it even offers any benefits over Windows+ayaspace.