Direct3D isn't the reason why OSX and Linux are second class citizens for AAA publishers. If those platforms had higher gaming market share publishers would do the work, but as it stands Steam is at 98.3% Windows users, 1.31% Mac users and 0.25% Linux users (http://store.steampowered.com/hwsurvey/). Linux has an extremely disproportionate amount of work involved for the user base, and Mac OS X is questionable for most titles. The Linux gaming audience is largely consists of tech people that aren't really gamers, but buy games to support Linux gaming, effectively for political reasons, this isn't a sustainable market (Humble Bundle helped indies briefly but the support has faded). Mac OS gaming is largely casual users that once again, aren't really gamers, and it has less to do with Mac's software stack (vulkan vs. metal vs. dx is irrelevant) and more with Mac hardware having underpowered GPUs for AAA games. PS4 and Switch don't support Direct3D, and plenty of big publishers port titles that make sense to mobile as well (neither major mobile platform supports Direct3D).

That’s never made much sense to me. I could see if you’re not using one of the major (portable) game engines—you might not want to spend the time developing Windows, Mac, and Linux ports. But if your underlying engine is already portable, would it kill you to hit build a few extra times to produce an “unsupported” Linux or Mac release?

Unless you’re deliberately using Windows specific stuff in you non-platform-specific areas like game logic. In which case facepalm.

EDIT: I know it’s not THAT simple but in general why not have a policy to be portable by default, platform dependent only when necessary?

The major cost barrier is support. If you are going to sell a product for a platform, you need to be able to have your CS staff handle support tickets involving it. Officially Steam on Linux only supports Ubuntu and SteamOS, so you can just throw out problems on other distros (albeit I'm not aware of any distro not taking responsibility for Steam breakages that work on Ubuntu and they aren't that common either).

Even then, having to implement that support infrastructure is time consuming if you have never supported that platform before. It is the first game that is the hardest. That is why publishers like Paradox and 2K can just throw almost every new release on Linux nowadays since they are all made on portable engines, are going to support OSX anyway, and they already have the support infrastructure for SteamOS and Ubuntu.

Its not even really an option, either. Since the games are commercial products in many (European) countries they have to by law support them in this way.

It's the other way round. Apple has never cared about games. Look at how the app store is structured. They'd aspire to be the "artiste" platform (aka photo-editing, music-creating, DJ, video-editing workstation).

That might be left in their DNA from the 80's when competitors like amiga got stereotyped as games machines and lost the much more lucrative business market. Computers where still dismissed as toys by many people and the existence of games on your platform didn't help.