Building a library or a framework comes with implied obligation to write extensive documentation. I wish more software engineers understood this.

I'm confident most software engineers I've worked with understood this. Usually the problem wasn't understanding that but rather meeting the deadlines and finding the resources to write that documentation. The only time I've seen documentation done really well, the company had a dedicated technical writer whose main job was making sure they had good documentation. Engineers would write the first draft and hand it off to the tech writer. My impression is that many companies don't want to spend resources on that level of documentation, which is the level you'd probably want to roll your own framework.

I meant it "part of the job" the same way unit tests are.

I see this more often when the project is open source. For some reason we feel like when its not, we don't need to pay attention to the documentation. Nothing could be further from the truth.