Because it isn't obvious to them? Some of that is because people engage with works only on a superficial level and actively refuse to engage with themes. Some of that is because bigotry is endemic and people don't notice things they already agree with. Some of that is because fans have an unhealthy investment in their work to the point they treat any attack on the work's text or background as a personal attack. Some of it is because they simply do not know the background of the creators or other contextual elements. I mean like "fan" is short for "fanatic" its not actually a favorable descriptor. You kind of answer your own question. Why would fanatics ignore something negative about a work they are fanatical about should be self-explanatory.
In any case though your attitude of "why aren't people outraged at something I dug around on and found a reason to be outraged about" to be rather tiring especially since a lot of is genuinely stuff that'll roll right past most people. Like people probably drop Highschool of the Dead for the gross fanservice well before they get into the far-right political party subplot. The stance that you shouldn't send money to creators who do bad things or whose games say bad things strikes me as an exercise in futility anyways. There is no ethical consumption under capitalism, all the companies you get products from are monsters by nature, and a lot of their CEOs and higher-ups are monster by nurture.
If you don't want to send money to people who are actively promoting alt right politics I can get that. If you want to promote better messaging in entertainment works and critique bad messaging, I can get that. But getting mad at, say, Halo because its actually kinda fash strikes me as a recipe for just getting hopelessly pointlessly mad all the time.