I'll take the step and say that most developers do the mistake to treat linux like any other proprietary system which they can target with stable interfaces and call it a day. Free software changes more often, but when it does, it is not that big of a problem because you have both control over code and the code that your code interacts with, because all code is accessible. If game developers opened up, and here I don…
This is akin to saying "When you're bleeding internally it's not that bad because all of your blood is still on the inside."
Games are a hit-driven industry. In most cases, it is strictly better to be working on a new game than to improve incredibly niche support for existing products. It would be one thing if fixing Linux bugs effectively future-proofed it from more bugs down the line, but that does not seem to be the case for most of the devs I know who've shipped on Linux.