It all boils down to the fact that C's memory model is meant to map closely to the physical memory model, and the C and Unix developers preferred simplicity of

It all boils down to the fact that C's memory model is meant to map closely to the physical memory model, and the C and Unix developers preferred simplicity of implementation over simplicity of interface. Leaky abstractions (like that the weekday or months start at 0 so that they can reference into arrays of names) are fine given that prioritization. Not everyone shares that preference.