just write complex expressions in a way that a human can most easily understand,
Unfortunately, (1) different people have different ideas of what writing is most easily understood, and (2) different readers have different notions of which writings are easily understood, and which writings are not so easily understood. To make things worse "easily understood" is not a boolean condition, nor is it necessarily well-ordered -- "most easily understood" isn't always a well-defined quality, even for a given audience.
Sadly the idea of writing in a way that is "most easily understood" has resulted in a race to the bottom, where writers are more and more encouraged to take the view that (some) readers are pretty much arbitrarily stupid, with the result that expressions become littered with scads of unnecessary parentheses that actually detract from ease of reading. Good writing is always a balance between too much and too little.