Yeah, it's interesting. The reasons vary for each company and books could probably be written for each one. A key factor is network effects. Read up on network effects: http://en.wikipedia.org/wiki/Network_effect

"In economics and business, a network effect (also called network externality) is the effect that one user of a good or service has on the value of that product to other people. When network effect is present, the value of a product or service increases as more people use it."

Not just internet and software businesses, railroads, telephone networks, and financial markets are other businesses that are impacted by network effects.