The short answer - because they are memes, and memes inhabit brains. Only when physical brain running the specific idea is dead, only then spreading of the idea is stopped. The bad thing, meme can be reimprinted into brain by reading a book. Think of books as cyst containers for memes. So you have to eradicate wrong books too if you want to stop bad ideas spreading. The question stays - is it worth it? Ah, yes, 21st century... You have to kill all webpages with bad ideas, including search engine caches.