Why is number of bits always(?) a power of two?

Question

We have 8-bit, 16-bit, 32-bit and 64-bit hardware architectures and operating systems. But not, say, 42-bit or 69-bit ones.

Why? Is it something fundamental that makes 2

Answer 1

Related, but possibly not the reason, I heard that the convention of 8 bits in a byte is because it's how IBM rigged up the IBM System/360 architecture.