You're not suggesting Apple only update iPhones every 2-3 years, are you?
If you are, there are two simple answers: That would kill them when their competitors are updating every two years. And, while it might make sense for individual customers to go 2-3 years (or longer) before upgrading their last iPhone, they didn't all purchase their phone the same year. So every year, there are millions of potential customers who last bought a new two, three, four, etc. years ago.
The smartphone market has matured. Radical innovations are harder to come by, and not necessarily what consumers even want: many innovations turn out to be gimmicks, and customers don't want to learn something entirely new with each new phone or OS upgrade.
Customers do appreciate incremental improves (to camera, processor speed, cell and WiFI networking, battery life and charging speed, etc.), especially as they compound over time. Are those changes "innovative"? The chemistry and engineering behind these incremental improvements often involves innovation, but the end product looks like an incremental improvement.