hardly anyone cares, hardly anyone is going to choose your product over a competitor's because yours has dark mode. For most companies this stuff is a distraction from the real work that matters. See also: achieving 100% lighthouse score
I can see why many would not bother. It means keeping at least 2 sets of colors going. Try putting green text on a grey background for example and you are going to have a bad time. It means making a lot of choices you already made and feel you are not going to get much out of it. I have seen many applications botch it. Thinking all you have to do is toggle the background to something dark and call it a day. It means probably means a redo of all of your art assets, and at a minimum what font colors you are using and making everything respect it.