Why are uintptr_t and intpr_t optional types in the C (and C++) standard?

With C99 (and later standards) standard reaquires certain types to be available in the header <stdint.h>. For exact-width, e.g., int8_t, int16_t, etc..., they are optional and motivated in the standard why that is.

But for the uintptr_t and intptr_t type, they are also optional but I don't see a reason for them being optional instead of required.