Because presumably France for instance would likely view someone blowing up one of their plants the same way as a nuclear attack. Given their nuclear deterrence policies that would end up badly for both sides

Germany doesn't have a nuclear deterrence and in the event of a nuclear war still might want to avoid having particular bad targets. I'd rather put any new money for nuclear into fusion instead of building large fission reactors.

A nuclear plant may be hit by despair, even if it isn't the target, and in any case finding who hit it may be difficult. Right now in Ukraine...