Why are Western politicians even bothering to react to Russian claims?

Some examples of claims from Russia which Western politicians recently had to respond to:

• Ukraine was making nuclear weapons in Chernobyl
• Ukraine had biolabs working on new biological weapons
• Ukraine's government is "fascist" and "full of nazis"

But why even bother providing a substantial response to claims like these? Why not simply say "everything Putin says is propaganda, there's no point in taking it seriously" any time someone brings up Russia's official talking points? It seems like completely denouncing Russia's words as illegitimate would be much more powerful than trying to take them seriously.