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.
Okay, let me have a stab at combining (albeit inelegantly) the best points from the other answers... I'll make this answer CW so that if someone sees a chance to improve the polish (or content) they can (plus I don't want to gain rep for combining other people's answers).
With DRM, you're giving people the means to unlock the content you've protected, along with the content itself. Someone's going to find that "key" at some point, thus defeating it.
At some point, you have to decrypt the content. If the hacker(s) can get access to this data then they've defeated your DRM.
At some point, you also have to show the content to the user and then he/she can simply re-record it "in the clear" from that data. See Analog Hole (This is less of a problem with games, as interactive content can't easily be recorded then interacted with again at a later date)
DRM only punishes legitimate buyers, because adding DRM is only going to reduce the scope in which they can use your work and thus makes them less inclined to buy it.
All it takes is one person with the skill, tools, and time to crack it then it can be shared with anyone and there's no point in buying your version (except if you don't want to do something illegal of course! (Or if you honestly want to support the maker)).
Many people will opt to use a free version of your product regardless of the legality of using it without your DRM, because you are imposing unreasonable restrictions on how, where and why they use your work. Spore is a good example of this, as are many other programs/games/etc.