I mean, most of the population isn't buying a new phone every year, it's just that there are enough people using phones in general that at any given time there are people buying new models. It's the same reason why there are people buying cars every year.
I personally use my phones for about 3 years. Sometimes up to 4, but usually year 3-4 is when the battery degradation gets so horribly bad and performance stutters so much that I figure if I'm going to do a full reset and buy a new battery and all that, I might as well get a new phone.