Software has essentially zero marginal cost. There's nothing stopping you, or any of us here, from writing another social network site. Management is the really hard thing to get right, if you want to grow to market dominance.

At its core, only Google's search engine is something of a moat. You have to crawl a sufficiently large chunk of the Web to yield good results.

You correctly point out Semiconductor Manufacturing as having a large cost of entry, which is true of any business that deals in atoms instead of bits.

Heck, just to set up a shop in your garage to make gears would probably set you back $10,000. But you could write the code to control the CNC machine, etc.. on a $100 netbook.