Just about any artist that does art like this (box art, fantasy book covers, etc) and is going for a realistic anatomically correct look is going to use a reference of some sort. Now, the more professional ones like Michael Whelan, Boris Vallejo, Rowena Morrill, etc etc will hire their own models, and either paint using the models as a reference, or shoot photos of the models, and use those photos as a reference. Its extremely rare to paint hyper real stuff from memory.
Now, in high school art class since nobody can afford any of that pro stuff, the class just usually goes to the library and finds photos in magazines and books to crib from. Obviously with these game box artists here that method never went out of fashion, or it could be just that it was the only financially viable way of doing this.
It certainly doesn't have anything to do with lacking creativity.
BTW, there is no way that Bloody Wolf cover used the Arnold pic as a reference. Other than the fact that its a strong dude in a tank top holding a weapon, there is no relation. It was probably stolen from one of the posters of one of the thousands of other shitty macho 80s action flics