Yeah, because of NUL-terminated strings. They cause so many problems it's not even funny. Even something simple like computing the length of the string is a linear time operation that risks overflowing the buffer. People attempted to fix these problems by creating variations of those functions with added length parameters, thereby negating nearly all benefits of NUL-terminated strings.

Why can't we just have some nice structures instead?

  struct memory {
      size_t size;
      unsigned char *address;
  };

  enum text_encoding { TEXT_ENCODING_UTF8, /* ... */ };

  struct text {
      enum text_encoding encoding;
      struct memory bytes;
  };

All I/O functions should use structures like these. This alone would probably prevent an incredible amount of problems. Every high-level language implements strings like this under the hood. Only reason C can't do it is the enormous amount of legacy code already in existence...