Tiny Phillips screws are not optimal for assembly. The Phillips head is designed to cam out before a screw can be twisted off, which is great of you are screwing boards together, but not really any sort of criteria you care about for automated assembly where the finished product needs to be blemish free.

This is why you see a lot of Torx heads on electronics and automobiles.

As far as the penalobular screw goes… yeah, that is I think deliberately weird, but when you buy your replacement battery or screen or whatever part, the company will either include for free or minimal charge the pentalobular bit, so it isn't really stopping you from repairing. What I think the bit does prevent is consumers opening and peeking around in their phone. Curiosity is great for learning, but it generally doesn't come with a grounding strap. The ICs in the phone are trivially damaged, not necessarily immediately destroyed, but set on a path to future failure by static charges too small to be noticed by the curious finger. Apple finds themselves on the warranty hook to replace these failed boards. It is certainly in their interest to keep sight seers out of the phones.