I'm not so sure about that last part. My guess would be that this is down to the difference in incentives between an open-source project and a profitable enterprise. In such a way that we may even
expect this to be the common case, rather than an anomaly.
Every doc writer Apple hires has to go on a P&L statement somewhere, their existence must be justified every quarter or so, etc. As others have noted, associating this work with the value it generates is notoriously difficult. Of course, some companies may have the kind of culture that understands the value of this work and keeps them around, but open-source projects can accept contributions from whoever offers them without regard for such bureaucratic concerns.
Basically, if you write some docs for my open-source project I don't need to pay for your healthcare insurance. The barrier to entry may also be lower for the same reason- if you write some docs, the pull request can be simply merged in. Hiring an employee is a much more involved affair than adding a contributor to the Contributors list in Github.