Because that's exactly what FXAA looks like
Look at that electronic panel in the second gif. The fact that you can even see the blurring in low resolution gifs speaks volumes. FXAA doesn't just smooth out the edges, it blurs textures as well, resulting in an overall blurry image. It just doesn't look good, and you lose image detail. And like with most post-AA solutions, if an overall image has a ton of aliasing, the resulting image with FXAA applied will be even blurrier than normal.
When AA is done right (like the SSAA that you'll find in offline rendering) it can clean up an image with minimal blurring, but if I'm being honest, I'd prefer no blurring at all, as any AA solution still results in some level of detail lost. But FXAA is just too noticeably blurry for my tastes.
Anyway, notice that I never used MK8 as an example of a Nintendo game that doesn't need AA. That game IS an aliased mess, and I'd rather it have some type of AA than not. However, it has so much aliasing that I think FXAA would just murder the image detail in the game.
Ultimately, if Nintendo continues to increase their baseline resolution with each generation, their games' need for AA will eventually vanish. The real solution to superior IQ is increasing resolution, not AA, and I do believe that Nintendo will find that solution worth pursuing, because there are gameplay benefits to a game having a super high resolution, like Pikmin 5 being able to render thousands of individual Pikmin, for example.