"You'll see way more stocks that are dramatically overvalued than dramatically undervalued. It's common for promoters to cause a stock to become valued at 5-10 times its true value, but rare to find a stock trading at 10-20% of its true value. So you might think short selling is easy, but it's not. Often stocks are overvalued because there is a promoter or a crook behind it. They can often bootstrap into value by using the shares of their overvalued stock. For example, it it's worth $10 and is trading at $100, they might be able to build value to $50. Then, Wall Street says, "Hey! Look at all that value creation!" and the game goes on. [As a short seller,] you could run out of money before the promoter runs out of ideas."
"Everything we've ever thought about shorting worked out eventually, but it's very painful. It's a whole lot easier to make money on the long side. You can't make big money shorting because the risk of big losses means you can't make big bets." - Warren Buffett
Notes from the 2001 Berkshire Hathaway Shareholder Meeting