Because it's not the feature checklists and specs, but the overall package that I'm purchasing. Since I'm already invested in the iOS and OS X ecosystem, and have seamlessly migrated multiple devices, the onus is on the competing model to offer up a total package that's clearly and convincingly superior to what I already have. Even if all things are equal, the iPhone wins simply because my apps, data, media, settings, etc. transfer over with minimal fuss. Going over to Android would require a lot more time and trouble, so the experience had better a LOT better in order for me to make that investment.
In my experiences with Android devices in general, and Samsung in particular, what they offer simply does not provide an experience that's superior overall, and in some key ways IMO is distinctly inferior. So, IMO things are not even at a point where it's a choice among equals. And many of the things that the S5 brings to the table, such as larger screen, removable battery, and SD card slot, don't matter to me. And in fact, I prefer the smaller iPhone size, and prefer its solid build feel over the thin plastic back on the S5 that's needed for battery and card slot access.
Also keep in mind that even if this whole "features and specs" stuff meant anything to me, which phone measures better between Apple and Samsung's flagship models tends to leap frog depending on the time of year. Right now, the comparison is between the iPhone 5s and the Galaxy S5. But, two months from now, Apple will send the iPhone 6 up to bat, and knowing their track record, it will have a sizable bump up in performance and likely outpace the S5 in most metrics.
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