I believe the primary reason is that there are counterfeiting counter measures, and if you laminated it, it would make it hard to use verification methods that allow you to make sure it’s a real one. They want to know that it is not copied, altered, or otherwise illegally fabricated.

Also, it should be noted that this is an identification card that can allow you to do crazy things like apply for official documents and loans. This number is extremely helpful for people wanting to steal your identity. I believe the idea is that if you lose it, it should degrade and disintegrate so you can get a new one without worrying that your identity will be stolen. Although, this is just my speculation.