Probably because they did not think of this special case when writing the standard, or did not find it important enough to consider complicating the standard text for.

In C89, there's just a general provision for all standard library functions:

> Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow. If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer), the behavior is undefined. [...]

And then there isn't anything on `memcpy` that would explicitly state otherwise. Later versions of the standard explicitly clarified that this requirement applies even to size 0, but at that point it was only a clarification of an existing requirement from the earlier standard.

People like to read a lot more intention into the standard than is reasonable. Lots of it is just historical accident, really.