It might seem hard to believe for younger readers—and as someone who spent about a third of his childhood staring at VHS box art, I find it pretty strange too—but there was a time when if you wanted to watch a movie, you pretty much had to go to a movie theater. There was a chance you could catch something on TV during that weekend afternoon timeslot if your local affiliate had picked up the rights, but second-run theaters were a pretty huge thing, and even mainline movie theaters would keep stuff like Star Wars in circulation for years because they were guaranteed to draw an audience.
In the '80s, though, the popularization of cable TV and home video changed the game completely—maybe even more than the introduction of streaming services in the 21st century. With a channel like HBO, viewers had access to movies pretty much constantly, and while you were still at the mercy of their schedule, a video store meant that if you had a VCR, you could watch whatever, whenever. With a video store, you didn't even have to shell out the money to own it. A couple bucks, and you had access to a massive library that, in the beginning at least, was so desperate for content that stores were willing to line their shelves with whatever they could get their hands on, whether they were major blockbusters or low-budget releases made on the cheap. Like, say, The Toxic Avenger. More on that in a second.
The practical effect of all this was that while theaters would make a token effort to keep minors out of R-rated movies, having access to all that stuff in your own home removed that gate and gave a level of privacy that would allow kids to watch anything, assuming they either had permissive parents or were good at being sneaky about it.
For a kid in the '80s, seeing an R-rated movie became a very achievable status symbol, something that was forbidden enough to be seen as cool, but not difficult enough that you couldn't pull it off pretty easily, especially if your parents weren't paying complete attention. I mean, if you haven't actually seen RoboCop, the VHS tape looks like it could be perfectly appropriate for kids. They didn't exactly put Bob Morton doing lines of cocaine with sex workers on the back of the box, you know?
And if we're being honest, it's not always about sneaking one past your mom. Kids tend to like action-adventure stories with loud noises and cool-looking characters, and seemingly "adult" movies like RoboCop, Terminator, and the later Rambo sequels all had that going for them. So as the grindhouse theater gave way to late-night cable and the video store, kids started to understandably gravitate to characters that were never intended for their demographic. RoboCop the movie might be a hyperviolent satire of consumerism made for adults, but RoboCop the dude looks like a toy.