To be fair, I think it's because they equate it with stealing. The propaganda works. Most people think that stealing is wrong, and media corporations have put a lot of money into convincing people that filesharing = stealing. Hell, even people who are okay with filesharing call it "piracy", because that's the corporate framing they were brought up with. (Just look at the name of this sub.)

Richard Stallman calls this out in this essay:

The term "piracy" is used by record companies to demonize sharing and cooperation by equating them to kidnaping, murder and theft.

I mean, yes, I enjoy saying "yarr", and "yo-ho-ho", and all the rest of it, but I have to admit that's the result of a successful propaganda campaign. Before people called it "piracy", they called it "filesharing". And, of course, that was a problem, because everybody knows that sharing is good and should be encouraged; but piracy is evil and should be stopped.