Because it doesn't make business sense to them. The author of the article makes just two arguments and assumes those are the only relevant arguments. There's a lot more involved in the decision to port GOG Galaxy to Linux. Like support, for example.
Personally, since proton got so good and heroic can just use any version of proton installed, I've began to buy GOG games again and run them through heroic. 99% of the time they just run OK. But of course I do my due diligence and check protondb before making a purchase.
Choose whatever is best for you.
That being said, as a Linux user I always appreciate a native Linux version of a game that runs well and is updated promptly.
As far as I know, there are game engines that make it easier to publish on many platforms, but I'm no expert.