Because they are less filling and taste great.
I researched the crap out of AAS before ever doing a cycle. To be 40 years old and see the kind of size/strength gains I made while on AAS was like being 18 again.
I pissed away my most productive years as a 20-something by chasing other dreams and never picking up a weight. My 30's were spent trying to fix ehat I had fucked up by pissing away my 20's. I never took the opportunity to try and reach my genetic potential wrt my physique.
It has only been in the last 4 years or so that I have become addicted to being as big as I possibly can. AAS has allowed me to catch up to my genetic potential while I am still young enough to enjoy being a big sumbitch.
But that is just my reasons - there are probably a million different reasons why regular joes like me get on the juice.