asian culture treats sex as booty for men. the man takes on certain obligations after screwing a woman, especially when offsprings are involved. marriages are extensions for the men's bloodline and territory.
western culture treats sex as entertainment. the man doesn't owe the woman anything just by screwing her. marriage is a just form of political and financial alliance that's separated from sex.
thus asian men like to claim exclusivity to their women, with obligation of full financial support, while western men don't care. in the western tradition, extra marital affairs were engaged openly and not even something to be ashamed of. it was like a big f*ck fest disguised under nominal marital binding.
Because most of European men cannot afford it. Also they like to try different each time and they are widely available. There is no need to keep the Mistress at all.