Why Does Disney Channel Shows Do This?

I'm Obsessed With Lizzie McGuire & That's So Raven... I Have Every DVD Out Of Both Shows But For Example There Is Only 34 Episodes Of Lizzie McGuire Out On DVD When There Is 65 Episodes Of The Series Total? Why Don't They Release The Series Or Every Episode On DVD? They Do That To Almost Every Disney Channel Show & Shows Like Bernie Mac Show & Malcolm In The Middle.... Those Two Show's Only Had 1 Season Out On DVD?