Why does pronom relatif eliminates a subject complement

Bonjour Ahmed,

Simply for the reason headlined in the lesson:-

"Relative pronouns refer to something previously mentioned. In French, the equivalent of whom/which/that can be que."

If we were to take your second sentence   "je ne l'avais pas vu depuis mon enfance."

The reader would be left thinking "Who or what is being referred to?" The second sentence is linked to the first with a relative pronoun so that the total text flow makes sense to the reader.

I hope this helps.

Bonne journée

Jim