Actually, hosting has been quite competitive for a while now. We need to keep in mind that back some years ago, while the companies were fewer, the number of potential customers was also lower than what we have today.
As for the existence of really low priced competitors, that's generally not a huge problem. See, even in a hypermarket, people don't focus on price alone. You'll see them going for the brand name, preferred color/aspect, for the most attractive package, etc. That's people using heuristics to make a choice when the alternatives are too numerous.
A growing marketing problem today for hosts is the need to attract customers form your competitors. At some point the new comers to the web world will become too few to sustain the growth of any company. Assuming that the customers of good companies have no intention of ever leaving, gaining market share will become harder and harder, as the low quality providers are slowly drained of their customer base.