You're a mage. You have the power to summon spirits, view, and VISIT, the astral, and can unleash spells from your fingertips. You're a rare commodity, in demand by every megacorporation, regular corp, and, heck, just about any other type of business to one degree or another. You could kick back, have a cushy life, with excellent pay, benefits, and relative safety.

But you, instead, choose to be a Shadowrunner, living from day to day, staying one step ahead of the law, being shot at (And hit!), dealing with sanity-destroying creatures and soul-destroying corruption on a regular basis.

Why?

Because you don't like being explicitly told what to do by mundanes who have no idea the vast complexity of your abilities.

Because the nature of Corporate employment is not conducive to independent research.

... Or because it's hard to hide those psychotic tendencies from the random corporate Mind Probes.

Or maybe you don't want to have the job of mind probing at least 30 prisoners per day, and good luck to you if you pass out on the job.

Because you want to blow stuff up but don't want to have to file the paperwork.

Because the magic, it burns to be free.

Because you are a greedy bastard and the possibility of a big payoff is more tempting than (even a big) regular salary.

Or because you just like the excitement.

Because someone out there is hunting you (the members of her old coven, in the case of one of my players).

Because cutting your cloned replacement hand off and throwing it at your therapist in the middle of a mandatory session doesn't bode well for your future as a magical researcher?

Because Gaia will have her revenge on those slavers that ravaged her, and you are chosen to aid in bringing forth her justice.
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Because then you'd be giving a lecture on the Unified Magic Theory to a bunch of snotty corp kids with less magical potential than a watcher-spirit now, instead of snortig novacoke out of the belly-button of this "exotic dancer" here.
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Because you see dem tusks here? Colleagues who only don't try to set you on fire because they know that you can kill them with your mind don't make for enjoyable working conditions...
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Because Wolf must runs with his pack. He stays true to his people.
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Because the average wage-mage has an average magic-rating of three, which is totally lame, and regularly gets him killed at the hands of slightly above average shadowrunners. And the only corporate-employed magician who is ever allowed to initiate is inevitably bound to be a major antagonist, which gets him exactly nothing but death at a major plot point, and maybe an extra edge-roll to bring him back at a later point in the campaign.
Nah, thank you, I'd rather run the shadows...

Vegetarianism.

Because the streets are all I have ever known. Sure those corp boys talk a good talk, but when it comes right down to it, this is my home, and nothing is ever going to change that.

*Edit*

Yeah, vegetarianism doesn't really go well with working for the Big A.

Because of the things that he/she did to EARN that criminal SIN?

Because being a shadowrunner is the ultimate powertrip for cybered-up magicians? Superhuman, supernatural, not bound by law.

Because your mentor spirit is Doctor Doom. Any cybersammy or scriptkiddie hacker can be a regular villain. Only an Awakened can be a supervillain.

Because the name of the game is Shadowrun, not Wage Mages 'r' Us.

Because my Dark Goddess won't let me stand idly by while some lowly wageslave gets the first cup of fresh coffee.

Because the little scheme of the other wage-mage that the Corps encourage did indeed make you fall out of the system, and you're not allowed to go to another Corp for work, so there you are...

Because I like it when they scream.

Because no one in the corp believed I was awakened, but I managed to fool the Shadowrunners

Because the Corporation didn't approve of my overly-affectionate Ally Spirit.

Because they said my overly affectionate ally spirit couldn't be dikoted, and now I'm on the run for luv.

My latest mage traces his roots back to the Mayans and has Criminal Sinner (Aztlan). He wants to undermine their efforts. Viva la resistance!

...because you had to disembowel your supervisor after he killed your puppy and used its bones in a ritual...and your mentor spirit is a Dog

...because magic is a spiritual calling, not just another toy for the powerful to buy. Magic comes from the soul. Working for the corps is selling your soul.

The Corporations just don't quite understand Toaster.

... because I've always been an independent with ambitions of the non-monetary sort, and magic only broadens personal goals of any kind. Maybe it's because the magic first manifested during something shadowy that I never thought of it as something to sell into another's structurings.

Because I am not GMing your games and your magician character has not been infected with a deadly toxin that requires him to get treatment every three days and only Aztechnology has the cure..

Because you cannot, will not, must not be contained.

What's the point of being able to shape the raw stuff of the universe if you have to punch a time clock and deal with everyone else's shit? Did Gandalf punch a time clock? Did Gargamel?

Well, he was just sick, and no one would hire him - pedophile, but that's not the point.

*flash to image of Gandalf standing in front of the Executive washroom shouting "YOU SHALL NOT PASS! .. without a gold card.*

Because in this game, only magic-types get the + X Sword of Awesome.
(Until Dikote returns and Sams get their +Xpensive Sword of Chopping.)

Because they took my Swingline stapler and I Ignited the building on fire...

...because they blew that whole "mind controlling the entire student body" thing way too seriously. It was just a community college! Yeesh.

because it runs in the family. And daddy ran the shadows.