Undefined behaviour exists because the creators of the C language wanted to leave compiler writers as much flexibility as possible to ensure that they produced

Undefined behaviour exists because the creators of the C language wanted to leave compiler writers as much flexibility as possible to ensure that they produced code that was fast. C was always a language that was intended to be used across multiple hardware targets and different processors have different behaviours. Restricting behaviour on certain arithmetic operations might require slow code to 'work around' the processor and generate compliant code. The creators didn't want that so specified as little as possible, leaving the rest to be 'Undefined'.