He uses because he wants too, simple as that:). He's probably gotten use to being Large, once you've been big like that, it's hard to give up.
Also, use of steroids is a bit of a double edged sword. Many old timers I've known layment that they got on a bit too early and wish that they stayed natural longer just to see what was possible. But a huge amount layment that they just don't enjoy training anymore when off the stuff. It's a mind fuck to see your total drop #300 or go from #220+ guy with single digit bodyfat to a #170 twink with loose skin or a #250 guy with 25% bodyfat.