It only does not have a significant adverse effect because enough people actually do pay for the media that they are able to make a profit off of it. If no one paid for it then they would lose all of their revenue from selling copies, which would definitely be a significant adverse effect on their profits.

I mean, maybe you don't consider that to be a problem. Maybe you think that copying media should be free and that instead of making money selling copies people should live off of the money they make from performances and/or patronage, even if this means that there is less money available to create media so in practice there is less of it around. I don't agree with this position, but I also don't think it is an inherently unreasonable one as long as you are being honest about it.

The point is, though, that whatever moral position you take on piracy, you cannot justify it with a claim that only holds as long as other people act differently from you.