Because a good chunk of "us" Apple people are at least as driven by what people around us will see. "We" want our friends to see our cool Apple stuff. I suspect a LOT of upgrades are driven by a "need"(?) to make it clear to even strangers that we own the "latest & greatest" from Apple. How awful would it be for those strangers to see us with only last years (or even 2 or 3-year old) Apple stuff.
I'm not endorsing it but it sure seems to be a buying driver for many.
There's a big crowd around here right now very worried about a number painted on a chip buried deep inside computers... chips that they and anyone else around them will likely never actually see. But oh, how we seem to worry about something having a 3 before something else has a 2... as if the world will melt down should Apple dare roll out anything with an invisible 3 painted on a chip before everything else has invisible 2s on the same chip.
When it is more visible, it seems to really press us to want to "upgrade." And if a perfectly functional design stays the same for too long, we start grumbling for a redesign anyway because "we're tired of the same old design" (but really because we can't show our friends we have the latest if their 2 or 3-generation-old one looks just like the brand new one we just purchased).
Again, not endorsing this, but its pretty much in every product thread every day.