arresting thieves doesn't stop others thieving, so why keep spending resources on it, eh!?
Because theft of physical items actually impacts people's lives in a real way. Steal a car and that guy may not be able to get to work. Nick a handbag and that woman may not be able to afford her weekly shop. Theft is also often violent. ... Download last nights Eastenders and ... umm ...

You can also have a wider effect than just punishing the perpetrators. They may well be resorting to theft for a reason other than just the hell of it. Addicts etc.
But yes, let's pretend that downloading a copy of something is akin to stealing physical items

I am in no-way defending piracy, content produces are just as entitled to the fruits of their labour as I am. All I am saying is that shutting down a few torrent sites is costly, and infective. It just plain doesn't work. At the very best, you stop people getting a new torrent file for what, an hour while they find a new site?