This is more startup and corporate hype than reality. Many startups pretend to set out to change the world to get funding, try to motivate their employees, and get good PR. The media is often eager to be fooled and jump on any bandwagon if there's a story in it, but people who've been in the industry a while and aren't wet behind the ears have heard such hype repeatedly, and it gets really old after a while, and is not very convincing.

Most people, whether in SF or anywhere else in the world, work because they get paid. Sure, it's nice to work on what you love, but relatively few people achieve that dream in the long run. Much more common is burnout and disillusionment, not just in tech but in many other careers as well. It often looks rosy and beautiful when you're fresh out of college, but gets less rosy with each passing year, unless you happen to win the startup lottery and have fuck-you money to really work on whatever you want whenver you want, or not, as the mood strikes you. Otherwise, it's mostly work, not play, and certainly not saving the world.