The hard part is not the modeling of the system.

The hard part is the interface of where the model and the real world meet.

The real world is fractally complicated.

Models require a concrete thing to interface with, but the real world is nebulous.

You have to resolve it down to absurd, unworkable levels of fractal detail to get it to be concrete again, for the model to integrate with it.

Those details cost increasing amounts of relative effort at strongly diminishing value.

Similar to the shoreline paradox. The closer you look, the more wrinkles you need to contend with.