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.
What Nasser using now is probably the most "off" for him, if you notice he's just maintaining for the past years, yet carry heavy weight probably more than 250lb.
i see no reason for that,
not being able to improve, injecting yourself with respectable amount of aas and gh (maybe even slin) while covering the physique from the world, so what's the point?