As much as I'd like to think that anyone can do anything, I don't think that's true. We've all got certain talents and limitations. And while you could train someone to become a CEO, that doesn't imply that they'd be a good CEO.
That being said, it takes less skill, but still a lot of effort, to be a good janitor. But, being a less skilled position, it's going to bring less pay. They're absolutely useful, and needed, but they're not up there with a good CEO. Or a good doctor. Or a good engineer. And that's why they don't receive as much pay. Now, there are certainly some places where I'd rather have the janitor in charge than the CEO, but that's just how things are sometimes... but most businesses can't survive with incompetence in leadership positions like that.
Now, don't take this as me ragging on janitors. For goodness sake, don't take me out of context here. I have a tremendous amount of respect for them doing a job that I wouldn't like to do, and don't do because I have other options available to me. I'm always happy to see someone out there, trying to make a living and working hard. I know that, if things had turned out differently, that might be me. There are a myriad of things that influence where we end up, sometimes it has a bit to do with culture and environment, and a large portion has to do with the choices we make.
I've been friends with the cleaning staff at pretty much everywhere I work, I know they're good people. As a human being, they're just as worthy as anyone else. In many cases, more so than the professional staff. But the job itself, not necessarily. It's not a knock on them as an individual. Nor should it be. That's where the elitism comes in.