Implementing this in the standard library means that the language needs to support pass-by-reference (which Pascal and C++ do, but C doesn't). This single feature does a lot to reduce readability. In C, I know that inc(x) is a function that does not modify the value of x, without reading any additional code[1]. In Pascal or C++, I need to look at the definition of inc() to know if x will be the same before and after the call.
An important idea at the core of readability for a language is the amount of code that I have to read to understand a single line. In any language that has pass-by-reference, this amount is larger than a language that doesn't. To achieve the same thing in C, I'd have to write inc(&x), and then everyone reading that code would know that x may be modified. (Note: the almost-equivalence of array and pointer types in C is a good counterexample where Pascal wins massively in readability).
[1] It could be a macro, but most coding conventions require macros that can't be used as if they were functions to be all-caps.