Because they are linked.

Is Japan's war in China related to Germany's wars? Not at all. But Japan's war with the Wester Allies definitely is. Japan only attacked because they thought the Germans were going to win in Russia (it's just a few months after Barbarossa), Germany declared war on the US just after Pearl Harbor.

To be clear, it's not a side front of Germany's wars, it's its own thing. If you see WW2 as only the Third Reich's War then yes it's completely distinct. But it's under the same umbrella because it is also linked to it. The participants are basically the same on the Allied side and Japan and Germany are allies... so it makes no sense to treat it as something wholly distinct. It's basically the same as the Eastern Front of WW2, it's clearly different from the Western Front, but still part of the same global thing.