pg has a good article on doing things that don't scale[0] and showed how the AirBnB guys went and setup the first listings manually, but it's actually far more general than just kicking off the network effect. Moats are things that don't scale. Open Source software scales perfectly. Billion dollar companies are billion dollar companies because they can use Open Source software wrapped around some Moat-able structure. The more Open Source Software that exists, the bigger the walled garden.
When Google spends a billion dollars sending cars all around the world taking pictures, they are doing so in a way that the Open Source world can't centrally compete against, as they don't have the billions to get that done. This means that all Open Source mapping software will be missing a crucial feature that users rely on, and will therefore go unused. But meanwhile the libraries that those Open Source Software use, Google can freely use within their walled garden. On the other hand, Microsoft paid a great deal of money to researchers with Encarta and had this same exact structure and Wikipedia came in and disrupted them. It takes more time for the decentralized approach to come online, but as long as it's possible and the data is kept high quality Google Maps will slowly lose market share to OSM.
Open Source software is generally created as libraries, whereas most companies build frameworks. Frameworks are easier to setup and give control to the makers, but they do not scale. Why is it that Microsoft Windows has 13 different dialog generations? Because each is a framework on top of a framework on top of a framework. It's amazing that they can even get that done.
On the other side, OSS is generally built on libraries. When the 2 UNIX devs were in a basement building UNIX and were able to out-compete Multics[1], they did it because they were building libraries that could talk with each-other using pipes around the boundary. Applications that communicate based on input-output with no internal state behave just like pure functions do. Pure functions compose. When Linus built git in 10 days, he was able to do this because the core idea of git isn't actually that much work. The library is built out of composable blocks that neatly come together. Microsoft's TFS Source Control is a framework that acts on your behalf and therefore the bigger the project gets, you need n^2 people to work on it.
You don't get paid for your value. The libraries create value but by definition that value goes to the user, not the creator. When you build a framework on top of a library, that value goes to the framework owner. Who owns the moat? The capital owners. Why don't you use your ability to create programs? Because the moats cost money to build and while you can create value building software, that value doesn't go to you. But by all means, make the world a better place.
[0]:http://paulgraham.com/ds.html
[1]: https://www.youtube.com/watch?v=3Ea3pkTCYx4, thanks to this HN comment(https://news.ycombinator.com/item?id=27494671) for this reference.