It is well known that steroids increase vascularity.
Mike looks like any top bodybuilder from the 1970s, when they used lots of steroids. He does not look like a top bodybuilder from the last 20 years, when they've been using lots of GH.
In addition, the vascularity and skin texture that you see in photos of bodybuilders, or on stage at bodybuilding contests, is a very temporary thing. Bodybuilders are only able to look like that for a few weeks or even days when preparing for a contest. The photos of Mike doesn't look like that because he's not preparing for a contest.
However, the difference in the look of a bodybuilder on contest day today vs one 20-30 years ago is again the drugs: GH, insulin and diuretics.