Putting aside legality, if you can go to the library and get a book and then press a button and get a second copy of that book that you can then keep forever, why wouldn't you? Hell, that's not even a good analogy, you don't really need a library card and you don't need to physically go anywhere.
This is the world we're starting to live in. People are arguing about whether piracy is right or good or whether it's evil or entitled or whatever. Here's the reality, though: We live in a world where there is no reason not to have a free copy of anything we want. That will not change.
And yet we still have a music industry. We still have a movie industry. We still have a video game industry. Books still sell. Porn somehow still sells, despite the fact that it is incredibly easy to find for free and there's so much of it because literally anyone with genitals (and without) can make it. In five or ten years, Wizards of the Coast and Games Workshop will still be making money even when 3D printers are matter replicators that require plastic. In fact, what harms these companies isn't loss of sales, it's the ease with which new materials can be manufactured. Ten years ago, fifteen years ago, making independent movies, games, books, music... that all required working your ass off, passing out copies for free, not having the resources you needed. Now? Steam, Youtube, Amazon, Kickstarter, IndieGoGo, Good Old Games, Vimeo...
Lost sales is not an issue. As other people have pointed out, piracy is not the same as lost sales. Studies repeatedly show[1][2][3][4][5][6] that pirates--especially movie pirates--are going to actually purchase more things. If you want a study showing that piracy actually harms sales, though, guess what, here's one stating the opposite[3][7] . When MegaUpload was shut down, revenue didn't increase, and for many movies, it was negative.
Ti West and others argue that pirating movies because they might be bad or they aren't available in your region isn't helping [8] . There's a reason it isn't helping, though, and it has nothing to do with piracy or pirates. It has everything to do with distributors and companies not keeping up with the times. Netflix on the other hand takes a different tactic: They find out what people want and what they like by actually looking at the piracy[9] . We live in a world where these old tactics do not work. Meanwhile Tim Schafer Kickstarted one of the largest projects ever [10] meaning the game made money before it was even finished being made. The Humble Bundle makes bank and it's a charity [11][12] .
We live in an age where things aren't as clear cut as they once were. Zach Weinersmith of Saturday Morning Breakfast Cereal has a career of drawing doodles on the internet. A good career[13] . Vlogbrothers John and Hank Green, and many more? Well, John is a writer, and Hank runs the DFTBA store, but they also are able to make quality educational videos like Crash Course and SciShow[14][15] as a career.
You can not afford to ignore this fact. And still many companies do. "Why would these big media companies spend so much money to stop piracy if it doesn't hurt them?" people ask. Why do legislators put forth sex offender laws that hurt more than they help[15][16][17][18][19] . People react to a perceived threat whether it really is a threat or isn't. No one wants to ignore piracy, or learn from it, because that would be giving in, or giving up. No one wants to be the one who risks it, but we're coming into a world where you really can not ignore it. The world is changing. Movie studios cannot treat piracy as theft, and they cannot treat it as lost sales.
Also, for the record:
At least rent it, even if you have 100 friends over to watch it. This is the sort of dangerous behavior that would make it impossible for smaller studios or independent film makers to make more films, or break in.
Pirating a movie or screening it to 100 people is effectively the same thing. In fact, many studios would punish that if they could, or at the very least do their best to stop it. In fact, that was one of the reasons the film industry was against home copies. They thought that anyone being able to watch a movie as many times as they wanted after only paying for it once, and to be able to show it to as many people as they wanted, would kill the industry. It didn't. And neither will piracy. Frankly, though? If it kills the studios, I think we'd all be better off. I'd much rather see proof of concept and then something like Kickstarter as the standard model for the movie industry going forward. Let the fans be the financiers. I at least hope that it would mean more people willing to take risks, and less stupid forgettable drivel like you see in blockbusters.
Washington Post: Study: Piracy actually helps small films make money
And just because: Humble Indie Bundle list with total raised
This post sort of got away from me. For everyone trying to say that piracy is bad and illegal so that's that, just keep this in mind:
This is bullshit - you're oversimplifying a complex situation to the point of no longer adding anything to the discussion.