Something about the early stages of the Korean War that I just don't understand

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What I learned in school back in the day was
that on a peaceful dawn morning, like a bolt from the blue, the North Korean forces launched a surprise invasion led by tanks, and the South Korean military, which had no tanks and very meager weaponry, had no choice but to be helplessly pushed back.
But thinking about it now, something doesn't add up.
First of all, at the time, South Korea's population was more than twice that of North Korea, and the economic power also had a difference of more than twofold, so there was a fundamental difference in scale. If they were pushed back to that extent, it makes me wonder what on earth they were doing until the outbreak of the war.
The most mysterious part is the fact that on June 25th, the day of the invasion, a significant number of troops were out of their units on outings, overnight stays, or leave, to the point that they couldn't respond to the war.
It wasn't as if the war suddenly erupted out of nowhere; there had been local provocations before that, and both sides were maintaining a state of extreme tension. In that context, letting the majority of soldiers out of their units seems like nothing short of madness.
Has anything been revealed regarding clear accountability for this part?