그런데 왜 this is the house (in which) my father was born

=this is the house (where) my father was born

=this is the house (that) my father was born

에서와 같이 where은 왜 생략할 수 있나요??????
이것도 전치사의 목적어 아니에요?

선행사가 the place 이면 생략할 수 있다는데 왜 저건 the place도 아니잖아요..똑같은 this is the house where he lives.
this is the house in which he lives.
this is the house he lives in.

여기에는 관계부사가 생략할 수 없다고 분명히 나와있는데
왜 위에꺼랑 도대체 무슨차이죠?