I'd guess it's because that's what C++ does. Though it's unclear to me that C++-style iteration (where you increment a running pointer until you reach an EOI pointer) is much used in C.

Storing two pointers into the same array is a memory safety problem. It's hard to implement a borrow checker when there are two live mutable pointers into the same memory object.

I would expect the borrow checker to treat a slice as a logical pointer to all elements that are addressable through it, regardless of how it's implemented under the hood.

This pattern originated in C; C++ iterators were specifically designed to fit it when it was already established.