If most companies make money by selling licenses, what compensating advantages are there for that lack of revenue stream:

• Supporting Software Investment can always equal zero
• Decreased Cost of Hardware Investment
• Free Developers
• Many Eyes Make Bugs Shallow
• Design and Security Review

They also have marketing advantages (since marketing is so expensive once you get over a certain size):

• Word of mouth
• International Attention on Open Source
• Code is forever
• Security

There are also a few problems unique to running a proprietary software company:

• Licenses are hard to write
• Enforcement of licenses is almost impossible in most parts of the world (including the US)
• If you want to compete in the big markets as a proprietary company there is a lot of competition.