A few thoughts:
1) With the tens of millions sold, the majority of iPhone owners probably are not really Apple fans, with any associated loyalty. To them, discussing other phones seems natural.
2) It's hard to escape a constant mention of Android and Samsung, simply because Apple is expending so much effort taking them to court. (Personally, I think this is backfiring, not least because of all the publicity Samsung is getting for free.)
3) People tend to stick with a forum they're familiar with. They feel like they're among friends. You see this with car forums, too: people might buy a different brand because of a new family etc, but that doesn't mean they want to leave.
4) iPhone (not Mac) forums seem to have a tendency to attract rabid fans who want to only hear good things about their favorite company. The more they try to make up myths, the more it attracts people who want to correct them.
5) Look at the thread topics. If there were no competing system threads, the iPhone section would be boring half the year because of Apple's slow release schedule.
I do agree that it might make sense to have an "Other Phones" subforum where comparisons and debates can be moved.