Why? Everyone needs to come to the network owner, and it isn't possible for new competitors to create their own useful network. The network effect is really the exact analogy for information-exchange businesses as physical capital was for businesses selling physical goods.

Except that in one sense the network effect is even stronger: oil wells can be divvied up by a trust-busting government (and will still function) but some networks only work reasonably when monolithic. This is an argument for a monopoly administered by the government (like for road infrastructure) or a private non-profit (like wikipedia).