Why do people brew their own beer? I am living in Germany and know at least 2 people that do it. But why would you take the risk if you could just buy very good beer at your local stores?
Hobby

Some people do it as a pleasurable hobby.  You might also ask why someone would build furniture if they could buy it, or why they play a musical instrument if they can just listen to the radio.

Personal Taste

You may like a particular style of beer that isn't popular.  And, especially in Germany, you may want to make a style of beer that you can't buy in the store - one that has other flavor ingredients, so it doesn't comply with the beer purity laws.