How can I explain why DRM cannot work?

The fundamental problem with DRM is that you're giving somebody a locked box and the key used to open it. You're distributing a copy of the key with the lock. Every person that possesses a protected Blu-Ray, DVD, software package, and protected CD also possesses the key that will unprotect it.

The people who design the DRM systems can try as they might to hide the key such that only those in the know (i.e. authorized decryptors/players/users) can find it, but there are a lot of curious people in the world, and all it takes is one person (or group of people) to be smarter than the ones who hid the key, and the box is open forever. As soon as one unprotected copy of the content exists it can be distributed everywhere, making the protection on the other copies irrelevant.

Digital files cannot be made uncopyable, any more than water can be made not wet.

— Bruce Schneier (source)

To sum the anti-DRM argument up in one easy word?

SPORE

How could a game with such intrusive DRM restrictions not be able to stop its excessive piracy rate.

If you wanted the hypothetical politician to understand why DRM wont work, don't give them a tech talk, give them a shining example of where it went wrong. One key point that 'management types' need to understand is that a pirated copy (DRM bypassed) is not equivalent to a lost sale. It just so happens that people are prepared to pay good money for products when they see the value in those products. "Copy protection actually increases rather than decreases the piracy of games." What left wing nut job said that?? It was only Gabe Newell from Valve. Ignorant companies are now competing with their own product, they now have to compete with 'free'.

When software is cracked (generally within the first day of release), DRM then only hurts the loyal consumers who paid for the product.

Side comment: A good quote I found on the Internet regarding gaming piracy and steam.

I'm not pro Steam/Valve, I'm just anti-stupid.