Battery replacement revenue and upgrade motivator.
Other manufacturer's phones do not last long enough to generate any post sales revenue through consumable replacements. Apple designs good hardware and supports phones reasonably long enough to at least generate one post sale revenue replacement (for those who don't already replace their phones frequently to generate another sale). Designing larger and longer lasting batteries will cut into those revenues.
Starting off a brand new phone with just enough battery to get through a work day results in the battery life degrading to requiring at least two charges a day after a couple of years of heavy use. As apparent in these forums, low battery life is a powerful motivator for people to upgrade their phones.
After a couple of years' heavy use? My iPhone 5 needed charging twice a day as soon as iOS7 was installed. It was only a few months old prior to that, and I used to get nine hours of usage time. The moment iOS7 installed I got four or five hours of usage time. The battery was fine, but thanks to iOS7 it will last half as long.