Writing documentation requires communication and collaboration with engineering. As you add people to an organization, you also add meetings, bureaucracy, and various other encumbrances. Not to mention that the technology to be documented needs to be done or done-ish, which is difficult when you're shipping major updates yearly!

Besides, how large do you think the talent pool is of good documentation writers? I would estimate that it's actually harder to find good technical doc writers than it is to find good programmers.

1) The minimum qualifications for a good API doc writer is practical experience programming with the technology to be documented. That already narrows the field significantly. You can't just a hire a comp sci grad straight out of college.

2) Indeed, a comp sci grad straight out college probably wouldn't want the job anyway. A lot of people aspire to be programmers, because it seems cool, fun, and has excellent compensation. Not many people aspire to become doc writers as a career.

3) The compensation for doc writers is generally lower than for programmers, so if you're already an experienced programmer per 1, then why would you take a step down?

4) The natural retort is going to be, "Just pay doc writers more!" But that's not going to work for several reasons. First, the programmers will be unhappy if the doc writers are paid more than them. Second, companies don't value doc writers as much. Third, once you get off the programmer career track and write docs, it can be difficult or impossible to get back on the programmer career track if you ever want or need to leave Apple. In general, the market is not huge or lucrative.

What you tend to get for documentation writers, then, is often older programmers who are starting to experience age discrimination, finding it harder to get a programming job, so they settle for writing API documentation. But that's not a huge pool of available workers.