No, that's really not how this ecosystem works. I'll give an example based on the Retroid Pocket 5, just because that's what I own and am familiar with. The company ships it with Android 13 by default. So this already has a full desktop environment, full suite of available app stores - everything that any other Android device can do, plus built in game input. No extra hardware needed. I can outright install Signal, Briar, Matrix, any of those kinds of things on it right now. Retroid never did any testing or development for that use case, it's just what happens when a device has a full, open Android experience from all of the development that has gone into Android and apps over the years.
However, they also chose to use a chipset that has support for Linux as well. At least two projects have already ported their variants of Linux to the RP5 - Rocknix and Batocera. While it's true that porting, testing, fixing, and maintaining operating systems has a labor cost, you're trying to apply standard business logic to something that doesn't operate that way. Batocera accepts donations, and Rocknix doesn't even accept any financial compensation. These are volunteer projects, labors of love. They are not subject to capitalistic incentives, and nor is it entirely accurate to call open-source projects like these "products." The processes involved are more organic, more democratized. It's a commons, and anyone can get involved.
The reasons they're designed the way they are is because that's what fits their motivations and project goals. It's the appliance model - they want the device to do one thing, and do it well - to play games, particularly retro games. So they put in only enough software to make that happen, and then try to make it as much of a polished experience as possible.
I am only trying to point out that the only thing getting in the way of more general purpose systems being available to these devices pretty much comes down to whether more people feel like trying to compile them.
