This phrase comes from a school textbook called Speak English 6e (for children in the first year of secondary school), first published in 1972 but popular throughout the 1980s in French schools.
Children were asked to listen to a tape which asked questions such as “Where is Bryan?” while looking at images in the book – and the answer they were supposed to give was “Bryan is in the kitchen”.
The next question was “Where is Jenny?” (Bryan’s sister), to which the response was “Jenny is in the bathroom”.
The phrase was later popularised in a sketch by comedian Gad Elmaleh who said he had been “traumatised” by having to respond to this “existential question”.
An earlier generation remembered learning the phrase “my tailor is rich”, which came from Assimil’s first English textbook, L’Anglais sans peine (Painless English), published in 1929.
It was the very first exercise and was probably chosen because it was easy to understand as the words look very similar to the French “mon tailleur est riche”).