Saying that it's not a matter of motivation means what? We don't have devs treat documentation as important and give up? No, like other tasks that devs do, we set up a culture of positive feedback (it's not always money) for caring about docs.
Exactly. If it's not evaluated and rewarded in the same way shipping code is (in hiring, too, as mentioned), it'll never be treated as important by developers. Devs are generally rational: if you say "write docs!", but then complain when things take longer because they were taking time to write docs, well, then you can guess what will happen.
1) no time set aside to keep it up to date.
2) the tedium of reflecting changes made in one place in 3 others.
3) the fact that the MVP of a lot of organizations is the go to guy who can answer all the questions while amazing documentation is quickly taken for granted & rarely personalized.