I don't think you can compare software with photography, as the open-source tradition in software is quite "old", you have the Free Software Foundation etc. Especially in the university environments and Unix operating systems this is a strong culture. I've made quite a few open-source things and not made a dime from it, like many many other before me. In parallel I've had paid work of closed-source software which is how I make my living.
The reason most make open-source software is not to make money, but it's a form of satisfying donation to the public, and if the software is free it's easier to get users, the "audience" to us programmers. It's also often the case that the development work that gives you money is "mediocrity" work, integrating databases and making yet another app, while with free software you if you want to really push the limits and just do interesting things which can be very satisfying. That's why user interface generally suck in free software, as everyone wants to be coding the complex algorithms inside.
But sure I can also get a bit annoyed by people's huge acceptance to pay a lot for hardware, and then nothing for software. Like buying camera gear for $50k and complaining about the cost of Photoshop...