What do you think is the reason the national team's defense has collapsed?

Korean football's defense has always been poor, but
it feels like it has truly fallen apart since the Bento era.
It seems like we are getting crushed defensively regardless of whether we are playing against strong or weak teams.
What is the reason?
1. Individual skill of the defenders
2. The midfield gets bypassed, causing the defense to collapse as well
3. Tactics
Which of these three is it?
The flanks have become a total disaster.