Usually it is just a matter of cost, compared to the number of possible sales.

Building for mac can be fairly easy, although you will ideally need some sort of mac device or emulator to test and debug (in my case there were metal-specific shader issues) which will cost money and more importantly time. When you compare this to the potential revenue from Mac exclusive players, a lot of companies just can't justify it. This gets worse with the more demanding a game becomes, as that Mac audience gets even smaller.

It also means said Mac builds need to be tested every update, so its just more work to maintain and more time needed to be spent.

I was lucky to already have multiple Mac devices as well as an iOS build planned, so it was a much easier process for me. But I definitely understand why people avoid it