Why can't a decrypted video be recorded from the monitor

Obviously I'm just a novice but I would like to know why a decrypted video can't be recorded from the monitor signal since it must be decrypted at that point to be clear and visible. I've tried to find the answer on Google but was unsucessful. It must be a simple but I don't know it.

What I'm trying to do is educate myself about DVDs. I've successfully made copies of DVDs I own using several of the programs mentioned in the forum but I was wondering why it was necessary to go through all that trouble when apparently the signal to the monitor has to be already decrypted to be viewed correctly. In other words I'm tryimg to find out for my own understanding of the process of encryption and decryption. The link you gave me seems very useful but I'll have to think about it for awhile. Thank you for your assistance.

Thanks to all who replied. It cleared things up in my mind.