A critical part of the human brain wants to overcome obstacles. We all know this to be true, because when we successfully overcome something we experience a distinct sense of pleasure. That is our brain sending a message to the body that "Hey yeah that was fucking awesome you should do that more because it make you even more fucking awesome and more likely to LIVE". Our highly evolved neurological systems have come to understand for themselves what is good for you, and this reward process is an iconic example of this. Recall now what it felt like when society graduated you after all that work, or when you experienced your first major professional success, or when you narrowly avoided that terrible thing because of a smart decision you made. So what does this have to do with beat 'em ups?
Because the right beat 'em up (I can't stress this enough; you have to be playing certain ones) is pitting your mental resources against a gigantic array of obstacles in the form of simulated opponents. Overcoming them means you have successfully utilized your mental and physical resources. This feels good physically because you have defended yourself (even in simulation) and your body wants you to know that.
Now this is where a lot of you say "But I've never experienced a sense of pleasure playing a beat 'em up that was non-trivial". That's most likely because you've only played them on the most basic level. You got thrown the beginner's hand and you squashed it. Which is awesome! But this is where I implore you to dive back into that super difficult beat 'em up you've never completed, and/or crank up the difficulty on that beat 'em up you really appreciate but never bothered to master. And when I say "master", I mean that you can express yourself with grace, agility, and power against whatever the game throws at you. That sense of pleasure you get defeating simulated opponents gets very real when you elect to go down the rabbit hole of difficulty. Play Devil May Cry 3 on "Dante Must Die" mode, or play Bayonetta on "Non-Stop Infinite Climax" mode, and do it until you're successful. I assure you, you've never experience anything like this.
You stop breathing. The game environment melts away into an abstraction of floors and corners. Enemies lose their textures and become silhouetted gray decks of mechanical cards. The audio vanishes. The only thing you can perceive is the motion of bodies and the rippling flare of frames-long weapon effects. Your body ceases to exist and you become the combat.
It's not the same as becoming one with the platformer or strategy game or sports game or driving game because those games are not representations of violent dominance, which your body understands on the most primitive and fundamental level. And while it may not be a real fight (which has profound chemical rewards), a masterful simulated combat experience is to a real fight what a roller coaster is to an orgasm.
Now jump in and see for yourself that I'm not fucking with you right now. It's enough to change you.