Are license plates that hard to forge? How much work is it to stamp one? (Not to mention that you can get away with printing plates even on paper. A buddy was driving his car intercountry but didn't have his official plate, just the temp ones. Border agents were particular about that part. A bit of Photoshop and applying a bit of dust and no problems at all during a trip spanning 3 countries.)

You can actually just buy one for 12 euros, with any combination you like. The stamp, at least in Germany, seems to be the harder thing. It is more common to steal a plate and punch them out.

When you think about it, although the stamps are not that hard to fake, stealing a number plate is a matter of a second. You just clip it out of a plastic frame.

Source: My number plate was stolen in Germany last year.