The original proposal at https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1406.pdf explains why.

> Some implementations have permitted anonymous member-structures and -unions in extended C to contain tags, which allows tricks such as the following.

  struct point { float x, y, z; };
  struct location {
    char *name;
    struct point; // inheritance in extended C, but
                  // forward declaration in C++
  };

> This proposal does not support that practice, for two reasons. First, it introduces a gratuitous difference between C and C++, since C++ implementations must treat the declaration of point within location as a forward reference to the type location::point rather than a definition of an unnamed member. Second, this feature does not seem to be used widely in applications, perhaps because it compiles differently in extended C vs. C++.