I am living in Japan, and have been for the past 3 years, the
answer is a mixture of both culture and weather.
Weather: Cold wind makes your lips dry easily, or is extremely
painful for your face in general if the cold wind directly contact
your face.
Culture: When they feel that they are too lazy to put on make up
(which is a thing here more so than in SG) they wear them to cover
their face. (Cold winter is harder to wake up to)