As an excercise - pick a random company out of fortune500, assume you have solid industry expertise (say - you've got a team of a couple of top guys who worked in that industry) in their area and also a couple million dollars, and think how likely you are to create a company that can compete with them. I'd say, that for a big fraction of fortune 500 (which is itself a large fraction of global economy), you need even more than the experise and a couple millions, which makes them fairly safe in their current positions.