For two reasons:
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The string meta data (e.g. length) is stored in the same memory block as the characters, to allow one string to use part of the character data of another string would mean that you would have to allocate two memory blocks for most strings instead of one. As most strings are not substrings of other strings, that extra memory allocation would be more memory consuming than what you could gain by reusing part of strings.
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There is an extra NUL character stored after the last character of the string, to make the string also usable by system functions that expect a null terminated string. You can’t put that extra NUL character after a substring that is part of another string.