'Because he is a fool' [to his masteres]

This novel is set in a Chinese village before World War One.
The protagonist Wang Lung lives with his father.
Today is the wedding day of Wang Lung.
He came to the very big mansion where his bride-to-be works as a maid.
Wang Lung came to the old mistress's place in the mansion, following the gate keeper.

Wang Lung fell to his knees and knocked his head on the tiled floor.
"Raise him," said the old lady gravely to the gateman, "there obeisances are not necessary. Has he come for the woman?"
"Yes, Ancient One," replied the gateman.
"Why does he not speak for himself?" asked the old lady.
"Because he is a fool, Ancient One," said the gateman, twirling the hairs of his mole.
[The Good Earth by Pearl S. Buck]
I think he should have said to his masteress "It is because he is a fool."
I'd like to know if it is right that they say to elders or his/her superiors a sentence without the subject and main verb.
Thank you in advance for your help.