Writing source code is fun.
Writing documentation and commenting code is less fun.
When a developer works in a company which enforces good comments and documentation, there is no choice: either this developer writes those, or he's at risk of being fired.
When a developer contributes to an open source project, he's doing it for free, and especially for fun. There is nobody to force this developer to do things he's not willing to do, like writing documentation and comments.
That's why many open source projects lack extensive documentation and comments.
How people can still contribute to open source projects with no documentation or comments?
If the source code is high quality, comments are not needed too much. The comments of public interfaces and the documentation are especially useful for the project consumers, i.e. developers who simply use libraries, not contribute to them.
There is no productivity factor involved. I'm working in a company where the actual codebase has no unit tests, no documentation, and no comments. I spend lots of time figuring out what a 600-LOC method is doing or coding things which are already done, but not discoverable because of the lack of documentation, so most of the time, I'm simply wasting company's money instead of doing something valuable.
On the other hand, for an open source project, it doesn't matter if one of the contributors wasted one week because of the lack of documentation or proper comments. The worst thing which can happen is that this contributor will leave the project.
Discovering code without comments or documentation may even be challenging, i.e. attract some contributors, instead of discouraging them.
In enterprise projects, it is not unusual for a developer to be forced to work on every aspect of a product, and, few years later, having to know nearly the whole system. On an open source project, nobody forces you to know the whole thing. You can simply contribute to a tiny part of it, and have an excellent knowledge of this part, without any need for documentation.