Apple can't realistically do board level repairs because they can't guarantee that a board level repair will bring the device back to its original level of reliable functionality. When you mess around with soldering and desoldering individual components, you can't guarantee the end result in the same way that you can when it's the outcome of a reliable and repeatable manufacturing process. For example, rework with a hot air station might imperceptibly weaken a solder joint somewhere else on the board, which will then fail 6 months later.
