D doesn't store capacity in each slice. This allows a slice to fit in registers more often. Instead for GC allocated slices, the GC will store metadata before the first element of the allocation. That does make it slower to access, particularly when the slice points past the first element, but it is cached to speed up repeated accesses to the same allocation. Repeated appending can be further speeded up by using a dedicated Appender type.

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.