So basically the biggest things are that:
We are conditioned to charge the smartphones once a day
They make money from not replacing batteries themselves and by selling them.
They are veterans in desigining battery enclosures that pass safety tests enough to maximize the battery inside.
Actually, probably the biggest thing is that original iPhone owners were pissed when they had to ship off their phones and go without them for a few days to have a soldered in battery replaced. It was a failed design idea. Now they have plugs so that the battery can be quickly switched out by Apple store support specialists.