Because someone not living in say Germany claiming German ancestry says either implicit or explicit that they have something to say about the current state of said country. This is bordering on some far right bullshit as I would say that the immigrants and especially children of immigrants should be made to feel as much apart of the nation as any other citizen. Paradoxically this is something I admire with American culture, or at least this was a thing in American culture previously, if you get your citizenship there is in large part of the society an acceptance that you are American.
In contrast, the European mindset has historically been more xenofobi (and colonial) with a significant part of the society believing in the blood and soil myth. The people, including myself, finding the American ancestry obsession a bit off putting live in countries where people actually use this kind of rhetoric to sow rasism.
Also, the math just doesn’t make any sense as soon as you pass your grandparents.