Ive been wondering though if it just seems the easy because we're used to it as a mechanic. Why is it not weird to quantify health as a single number? Why that and not happiness or friendship or awareness or heck, money?
Its definitely satisfying now but I think we had to get accustomed to it
I think there are several reasons, actually.
A good dev friend of mine was often criticized for his saying that "videogames are all about collision detection." Now, obviously this is oversimplified but he has a point. When you get down to the nuts and bolts of a game, beyond all the high-falutin' "themes" and "meanings," the gameplay often does come down to just that. It's abstract.
And in the abstract world, it's very difficult for us to model intangibles, such as emotions, as a discrete gameplay element. That stuff is generally flavor. So when you think, okay, our game mechanic is about going from Point A to Point B, what is going to stop us and thus make it challenging and fun? The environment by itself, maybe, but generally that's not very fun, or it gets boring after a while, and environments are typically static. Well, how about putting in some agents? What do these agents do? Well, they try to hinder you somehow. What hinders you? Etc. etc. until you get down to the solution of, "they do damage, or they kill you, and you have to avoid them or kill them first." Again, collision detection
So ultimately I think it comes down to convenience and clarity. It's really hard to model a realistic emotional system that lets you address things in a way that feels good. Could you make a game where instead of reload, punch, shoot, etc, you did yoga to recharge, hug, and give candy (actually a boy and his blob does like two of these, lol)? We probably could. But it might not be very exciting.
It's a very, very tough problem!