Hello.

Maybe this has been answered before, but since ScummVM is developed with portability in mind, why aren't all games distributed powered it released for mac/linux as well?

Regards,
m

Most probably some legalese issue, or a lack of willingness to provide support for Linux users.

Whatever the real reason is, your suspicion is right: it would require almost no extra work to provide installers shipping with a Linux version of ScummVM. So the answer is not technical.

Well, MacOS is simple enough given how transitional Big Sur made things; but otherwise there appears to be nothing stopping the Linux build now. As far as I can tell, whatever issues were with games like Myst III were fixed.

(I do wonder how much dev time they're wasting on ports to things like the FM Towns, though.)

Quite a lot, I would hope.
DOS versions, and some other platforms, have been around for decades. But the ability to play FM Towns versions of some old adventures is something new, at least outside Japan.

One of the biggest selling points for Zak McKracken on GOG is the ability to play the FM Towns version which very few people have ever tried, many have never even heard about.

If you mean exactly as you wrote, porting something TO FM Towns, then I actually don't know what you're talking about...

From Towns, fair enough. To Towns, wouldn't be the craziest port they've done.