Some types of complexity have well formed mathematical definitions. Cyclomatic, time and space, kolmogorov, etc.

Something that's complex to the intellect is ill defined. I suspect it can be defined, but it's very highly dimensional. That is there is no scalar complexity number that can be used to compare items directly, and instead you may have two solutions of equal complexity but in different (and otherwise incomparable) axis.

The issue we have today is that we have no academic way to approach complexity so people just sort of develop an intuition for the subsets that they encounter and get bitten by.

However, because they have only a partial picture, they end up simplifying the complexity they do understand in a way that increases the complexity that they do not understand (at least until it's too late for the project).