It's pretty simple: those brands that are advertised so much are typically commodities, meaning they are pretty easily replaced by another brand's similar product. So they want to stay top-of-mind, so that when you reach for a soda, you remember Coca-Cola.
Beyond that, YOU may know Coca-Cola from seeing thousands of ads for it over your lifetime, but the impressionable 14 year old next door might not. So they want to keep pounding that message ("Coke = Happy") into his head, too.
In the end, the more replaceable and less important an item is to consumers, the more the company is going to push it to a mass audience. Switching from Coke to Pepsi, PC to Mac, or even Chevy to Toyota, is ultimately not that tough a decision; but you don't see Twitter advertising much, because they are fairly unique in the market and they're already top-of-mind when you think of "social network / microblogging platform."
Remember: Your $1 may not mean much to Coke, and even your $5,000 lifetime revenue might not mean much, but if 10,000 people like you suddenly switch to Pepsi, that's a lot of carbonated sugar water that they aren't selling. So they're going to keep pounding away with ads as long as people keep buying their products.