1 - a lot of grads have loans to pay off, and that $100k/yr job sure helps get it done quickly so you can focus on anything you want without that on your back.
2 - That $100k/yr job can help you meet a lot of great people to work on projects with at a later time. A lot of SCS grads come out having done nothing but stare at code for 4 years (I'm one, and I was saddened by the number of my classmates who made no connections on campus outside of SCS, connections that would be vitally useful in a startup). Working at a great company in the valley can help get you into the startup scene in a way that Pittsburgh might not.
3 - Nothing says you can't work on a startup outside of the 40-50hrs/wk you put in at your 100k/yr job. SCS grads are used to loooong work weeks; a normal work week seems short in comparison, especially right after graduation when you aren't (probably) supporting a family or anything else. That leaves time to make money AND try out some projects.
4 - Not being part of a startup =/= slaving your life away. There's a lot of great, rewarding work to be done in the valley and in the tech industry, it isn't an all or nothing thing with the all being becoming a startup founder. Being part of a company growing quickly and doing cool things can be just as rewarding as being a founder.