Linear is easy, non-linear is difficult to calculate
An ideal, absolutely linear system does not exist. It is enough to consider the ignored factors and the approximations used to expect that such a simple system of relations will not be valid without restrictions. Still, such systems are the easiest to calculate as our entire system of algebraic mathematical concepts and all related operations are derived from simple counting.
Mostly the models are separated into sections for the purposes of the calculations, and it is continuously checked whether the differences from real relations within these sections go beyond the limits set by practical requirements.
There are methods for handling nonlinearities as well but they are unique methods which cannot be generalised.