That's because making a big pile of money the Silicon Valley way requires two things: a product, and a moat. It's not hard to create an email product, but email moats are hard because it's an anarchist technology by design.
Warren Buffet talks about "moats" all the time. Basically a business with a moat is just a business with a defensible advantage.
If you think of your business as a castle, a moat is something that stops others from taking your castle from you. This could be anything from a network effect (Facebook, LinkedIn), economies of scale (Amazon, Walmart), to patents (most biotech companies) to brand (Coke).
The way he put it was something like, "If you gave me 100 billion dollars to take Coke's market, I can't. They've been investing in infrastructure and brand all over the world for decades. Even if I could make a better tasting beverage, people would still buy Coke. They have a moat".
A moat is a competitive advantage that's hard for competitors reproduce or otherwise overcome. Coke's moat is its near universal brand awareness and affinity. Comcast's moat is its infrastructure and political clout. Crossing a moat in any conventional way is very resource intensive and often leaves the challenger vulnerable.
