Some of them also evolve over time or are part of a certain urban culture, I am sure there is a lot of slang used these days by the kids at school that I wouldn't understand. Some expressions I think are created spontaneously by some media events or programs and can take on a life of their own. I don't think though they are that important for effective communication. As a little test here are some slang expressions we use where I am from (the western part of Germany), let's see if our Austrian friend is familiar with them:
If a situation seems hopeless without a way out we might say: "Dann ist Hängen im Schacht.", or "Dann ist Holland in Not."
If a problem has been solved we might say: "Dann ist der Drops gelutscht."
An example for an expression that just sprang up a couple of years ago and ended up being used as a set phrase, usually expressing some sarcasm is: "Dumm gelaufen!" Again, languages are living organic things, constantly changing and evolving.
A little post scriptum here. I just googled "deutsche Sprichwörter" and found this website http://www.alle-sprichwoerter.de/
. I looked up under "F" and did not know any of these sayings, I hope no German learner will use these lists!