@Arkandel
I'm going to employ a bit of fiction logic here, but follow along.
Cyberpunk is developed from the ethos of noir. One part of cyberpunk is that everything is disposable, including people. Especially people. This is why the stigmatism on wage-slaves or salarymen. Shadowrunners started as people you would hire when you needed dirty work done, people who work beneath the law or able to be flexible about it. It's kind of like hiring a Raymond Chandler private detective; they might get the job done but it's a dirty job that nobody really wants to do.
In the cyberpunk future, things tend to be worth more than people and the cost for installing and maintaining cyberware doesn't give you much of a chance to strike out on your own; you're owned.
Now Chandler was more hard-boiled than noir, but Gibson was writing about people, and generally he was writing about the moral ambiguity of people that would eventually lead his heroes to do the right things in spite of the harm they might do to the egos of others. New Rose Hotel is the seminal example of this, but pretty much half of Burning Chrome is solidly noir in its approach on human relationships.
But ShadowRun is a game, and as a game you can make it about anything, but on the whole I believe 'runners are not rich because they're paid for shit or at least their payments have to almost immediately go to paying off everyone who got them there, fixing things broken. c.f. Listen Up You Primitive Screwheads, which is a wonderful guide how to fuck up your players' characters' lives in a fun and cyberpunk way. More punk than noir, absolutely. CP2020 is all about the punk.
Which brings me back to my original comment: I don't know what ShadowRun is supposed to be about. I don't think it's cyberpunk. I think it's post-modern fantasy.
tl;dr: Because life sucks if you're not protected by The Man, and life sucks if you are.