Orkhepaj: it is still an extra work , maybe they don't know how to do it and just don't want to learn it
Depending on the tool you are using, not that much extra work.
I recently set out to create command line tools in Golang and found out that Windows and MacOS are supported out of the box.
I could be a jerk about it and say I won't spend 30 minutes to create a compilation pipeline for Windows or MacOS, but when its that simple, why not aim for the stars? I mean, I can even compile the bloody thing for 32 bits Windows and someone is actually using that. That's progress.
I get that things with graphics are more complicated, but some of those game development frameworks like Unity are very mature at this point and have been supporting multiple platforms for quite some time.
Again, if you are not a Microsoft or Apple employee, that's the desirable outcome.