If you're a search engine and I send you a billion takedown notices, of which one is legitimate, and you don't comply with that one, then you will be liable for infringement. Is it worth the risk?
This is similar to the problem where YouTube content claims cause all video revenue to be diverted to the complainant, before any review has actually occurred and leaving no recourse to the victim. There's literally zero downsides to filing the claim, so why not do it? Why not claim everything?
Unrelated: I wonder why the textarea for posting comments on HN turn orange when you click on it? It is really annoying....