For a lot of open source frameworks and libraries it's not a financial decision. We need them for what we're doing and we're happy to share them with others. If we're lucky then we'll get some help from others on them.

Some libraries that provide an interface to a commercial system will be given away free by the company to make it easier to get people using the system. Once they're using it they can then be converted into a regular income.

It can be rather good for your career if you manage to write/run/maintain a reasonably major open source project. So for some it's basically an investment.

'cause it's fun.

A lot of them are backed by corporates. Whilst you don't pay - someone does. There's no free lunch!

I've released two frameworks and a bunch of libraries over the years for free. I can't say why others do it, but here's why I've done it:

They were things I needed to develop for other projects I was working on (some for profit, others not). I found them very useful and thought others would, too -- so I released them.

Could I have sold them? Probably. But I was already paid for these because I used them in projects that I did sell, and I prefer to release useful software to the community when I can afford to. It's better for everyone that way.

Also, there's the question of support. If I sold them, I'd be obligated to support them. That's an additional cost that I didn't want to commit to.