So that people that buy that year can get the latest improvements

They're not releasing them on your personal upgrade schedule

Because even if your phone only breaks every 5 years on average do you want to get a new 5 year old phone because your last one died right before the refresh cycle? Having regular releases means that when you do get the phone you have a relatively up-to-date device with latest hardware improvements. You don't need to upgrade yearly because they release a new phone yearly.

You probably don't, as your current phone ought to last for years. But hardware manufacturers, like software developers, benefit from faster release cycles.

People are on different phone cycles. Even if most people update once every 5 years, it only takes 5 distinct groups to warrant a yearly new phone.