Comment by cryptos

What sometimes gets forgotten is that even a programming language _is_ a model! And it is a model that fits exactly all the details that are necessary to construct an actual program with all the little special cases.

Modeling all constraints and runtime behaviors in UML is cumbersome and hard to understand. UML could be used for showing larger building blocks or complex flows (e. g. with sequence diagrams), but it is a bad fit to model a complete program in it.