Compounding the mystery is that it crops up everywhere, not just electronic noise plots. But outside of a few systems such as electronic noise in a laboratory setting, it's phenomenally hard to measure. For one thing, to get into the 1/f domain, you have to measure things for a long time. And the noise measurement itself is noisy. And the number of things that you need to control, such as environmental conditions, increases.

So its existence is often largely treated as an empirical rule of thumb rather than having a specific physical cause.