It is a remnant from the Northern Song Dynasty, as a form of etiquette and being polite, to not to disturb them while eating.
"凡往见人,入门必问主人食否 […] 度无所妨,乃命展剌。有妨则少俟。"
“Whoever you visit, you should ask them if they have eaten or not […] if they are about to eat, don’t enter the house until after they finish the meal. Otherwise, you would be intruding.”
From the book "Lü Convention" 《吕氏乡约》
Time to live in caves again, best natural insulation