Because they're designed for different things.

Ampere's primary focus is running lots of simple tasks concurrently, at relatively low power, with lots of I/O. So, many tens to hundreds of cores, not too fast, at lower power draw than amd64, with lots of PCIe lanes for storage and network.

Apple's primary focus is user experience and power efficiency. That's why you'll find a handful of fast performance cores and low power efficiency cores, along with graphics acceleration to drive high resolution displays.