Airlines may have pretty big revenues, but their profit margins are tiny, and at the end of the day, net profit is what matters to investors. In 2010, Google only made $30 billion in revenue, which is only a tiny bit more than Delta airlines made. But Google is worth $184 billion, while Delta is only worth $8 billion.

The costs of servers scale extremely well, while the costs of airplanes and jet fuel don't. So web companies have much higher potential profits than airlines, and they're consequently worth a lot more.