You've got the rights answer at least in part. It's also copyright holders sending out bulk notices that even get the IMDB and legal streaming links taken down. They claim the notices are automated so it's not intentional but it doesn't change the fact that the links get censored. Marketing teams for remakes of movies and television shows will often intentionally get legal links to an original film or show taken down so anyone searching on Google only finds the remake version. There are also plenty of films and shows that despite being public domain get taken down by fraudulent DMCA notices. I read somewhere that copyright related censorship has led to nearly three quarters of the available internet in the world no longer being indexed by Google. Well, it's still indexed but the public will never see it.