I can sing along to korean songs (or other foreign languages) but i obviously can't actually have a conversation in korean
I think speaking is just harder than singing
for songs the melody and pronounciation gets stuck in your head after few listens (they have guides)
and at least in my case i tend to follow the romanization of the lyrics instead of actually reading them in korean. not sure but i'm guessing some artists could be doing smth similar to that
plus like everyone mentioned they have enough time to rehearse their lines for the song and retake them if they miss the right pronounciation
for interviews, even if they are scripted it takes more time to remember words than melodies so they need little time to think (probably why they sutter or talk slowly sometimes) and you obviously can't refilm an interview over and over again to fix pronouncing mistakes