I think a lot of Europeans just genuinely don't make the connection between citizenship and actually being the nationality/belonging to the group. I guess it might be a historical left over from the early nationalist days. There were (and to a much lesser extent still are, I guess) nations without a state, so also without the means to grant citizenship. Like, there was a Polish nation in the 19th century, but there was no Polish nation-state and thus no Polish citizens. Hence, being Polish can't be defined primarily through citizenship.

A lot of governments do have weird citizenship laws, though a lot of them seem to be pretty old from what I know. Still, the idea that blood is what makes you French/German/whatever is generally considered to be a sign of someone being pretty far right or at least very nationalistic. Worst case they may well be a fascist. (at least here in Central Europe, not sure about the rest)