iPhone chips have a long lifecycle and a large user base.

The iPhone is one of the most popular devices in the world, and they have a 5+ year lifespan between yearly upgrades to people who buy secondhand years later. So an iPhone 14 chip will find use through 2027 or later.

The iPhone chips periodically also end up in lower end iPads, Apple TVs, HomePods, etc.

Beyond that, the engineering efforts impact all apple silicon, including Mac, so the efforts can apply to more devices.