Viral hits are a species of their own, not even Psy could have predicted GS or can replicate it.
No one knows why, which song can attain such global vitality.
With all respects to Psy, BTS has built far more a consistent portfolio of quality and acclaimed work.
Sure the freak vitality eludes them, but that's like winning a lottery, very different from being rich by creativity, perseverance and enterprise.
This is like saying why Elon Musk has not yet won a lottery.