You make a fair point here. Maybe bts do take extra caution when dating.
But for the groups they are the only really popular group that never got exposed. Current really popular (bp-twice) and past really popular (bb-SNSD-exo) all got exposed.
probably BTS are just really cautious. I don't see any other reason. Also think about it, they were never exposed even when bighit could never have enough money to pay dispatch. I doubt the reason that they didn't date back then. Their career could've ended unlike big3 groups if their dating news was revealed before the international fame. They always had to be cautious of every step.