Apple is designing for more IPC and lower clocks, whereas Intel and AMD basically need to target their designs towards a 5GHz target clock rate to remain competitive in the marketplace. A lower clocked, but much wider core can be much more power efficient, and Apple almost never provided sufficient cooling to run Intel processors at sustained top speeds anyway. A larger die increases cost, but vertical integration means you don't have to find margin on each component, just the assembled product.

E cores work the problem the other way, if you narrow the core, you can get about half the work done per core, but in about a fourth the space. That's a win for throughput, but not for latency/interactivity. Drastically different per core performance makes OS schedulers work harder though.