People are giving you the starry-eyed entrepreneur version, which builds its rhetoric on how they're "changing the world", how they're motivated by learning and pushing the boundaries of what's been done and what is possible, and so on and so forth.
Here's a counterpart explanation that some may consider a bit more cynical, built after some time in SV working for early stage startups, advising some early stage startups, and even founding (!) such a startup.
The short story is that all of this is essentially a giant economic Battle Royale of sorts, with VCs and investors acting as puppetmasters at the top and raking in all the actual money. When you have an entrepreneur such as Zuckerberg or Page/Brin who emerges at the top, it's purely an unintended side effect of this. The people who are really bringing in the big bucks are VCs/investors/incubators/etc (with the real estate and media companies coming in in second place).
Silicon Valley now is a far cry from the Silicon Valley of the 70s-80s that a lot of people have in mind when thinking of startups (and even then, I have feeling it may have been oversold- but I wasn't there). SV today is an extremely large industry, with lots of money to be made, where every startup is touched in some way by the underlying finance behemoth.
The procedure is simple: take bright kids in their late teens/20s, sell them the dream that they are the smartest in the world and the intellectual elite (the easiest way to get them in that mindset is to get them to apply to be part of a super exclusive, hard to get in club- sorry, "incubator"), that they too can change the world with just some Javascript code, 14h days filled with Mountain Dew, and a $500k seed round. Let them battle it out on the market, and acquire customers any way possible. Most companies will die in less than 24 months, often in shitty circumstances (users lose a product they may have relied on in less than a few months' time, employees get fired with no notice or explanation some random Friday afternoon, etc.).
But the 0.1% of startups that makes it out is more than enough for investors to get back many times what they put into all the other startups (cf. YC numbers, for instance), so this doesn't bother the investors in the slightest (they can give out dozens of $500k seed rounds and not have their wallet dented in the slightest. They basically have infinite money in a roulette game where the top prize is a Google or a Facebook or a Whatsapp).
There are a few older founders who know how to play the game and how to actually turn it to their advantage, whether good or bad (for the good kind: cf Nest. for the bad kind: cf Color Labs). But by and large, SV now is 20 years old slaving away for the promise of "making an impact on the world" and "being the next Mark Zuckerberg".
The good news is that if you're aware of all of this and not too dumb, you can make a good living in Silicon Valley (be sure to save money on each paycheck no matter what), and it can be a great start to your career. The bad news is that it's also easy to fall for it, and find yourself at 35 having worked only for early stage startups that have failed, leaving you with 15 years of work and very little in terms of these silly little things some people like to call "career growth" and "retirement plans". I've seen engineers starting to hit the mid-late 30s, realizing that they're not good enough to make the switch to finance/management or to a larger, more secure company. Some of these people move away to other cities where they can aspire to a more stable career (taking a major pay cut, but gaining things like a 401(k) ), while others keep playing the startup game hoping that the next one will be the one that brings them $10M in options and a VP title.
Caveat emptor.