Just because one person is willing to do everything needed to test on all platforms doesn't mean everyone should therefore be willing to put the time and effort into it.

Depending on what tech you use it can be easier or harder to do as well. I'm making a game with Love2D now, which has made supporting Mac and Linux rather trivial so far, although I've run into challenges with mobile platforms and the web even though it support them (it does work, but it takes more low-level glue code to support native phone features, and the web doesn't seem to be well maintained and my game is throwing webassembly errors currently when I try to run it).

And my previous game (which is on the backburner for now) was made with Monogame, and while that technically has support for Mac and Linux (as well as mobile), I've had quite a few issues even just getting the Mac version working well, like issues with resolution, rendering 3D properly, getting shaders not to break, etc. And they haven't kept up with the latest Mac updates the past few years and have had to make a special push to try to get it caught back up. I've probably sunk a good 20+ hours trying to get it working well before putting that aside to work on the actual game again and I still might have to rearchitect things a bunch in order to get that working.

Meanwhile Unity would probably be pretty dirt simple to port, for the most part, but it comes with other tradeoffs, like not being open source, and trying to pull a stunt a couple years ago where they pulled the rug out from under developers with changing licensing to something aggressive (that convinced other developers to port their games away from the platform), etc.

And there's Godot, which seems to be getting more support again (which is great, I even considered it or my current game, I just like coding in Love2D a bit better), but if you ever want your game on consoles you have to pay a third party to port your games to consoles for you.

The guy you linked makes their own engine (and to be fair, so does Jonathan Blow, who you're critiquing), which is great, but not everyone wants to get that low level. I would rather spend more time focusing on building the games themselves, which is already hard enough, rather than spending all that time building an engine.

It was for that reason that I spent several years focused on board game design instead (as I can make a working game with drawing on index cards and some colored plastic cubes in less than an hour), although that has its own frustrations with significant hurdles to get your game signed by publishers as an unknown designer (I did get one signed four years ago, and it's still not released yet), and large financial risks being made for manufacturing and distribution.

Edit: Also the person you linked to isn't even sure it was financially worth it to support all of those platforms, they just do it for other reasons:

"Do you make your money back? It’s hard to say definitely which could mean no. Linux and macOS sales are low so the direct rewards are also low. ...For Smith and Winston [their current game] right now I don’t think it’s been worth it financially (but it will probably make it’s money back on Launch)"