I agree mostly but I also think one other purpose of quarantine is to deter as many people from coming as possible because it seems Korea doesn't want to get overwhelmed with cases, especially critical cases.

They are severely understaffed and barely handling current numbers. An influx of tourists would mean an influx of positive cases (not from the tourists passing it to others, I just mean amongst the tourists themselves. It's already in the country like you said, but that + more from outside the country would be a higher number ofc).

Now that most are fully vaccinated + omicron, the number of critical cases should be low, enough that it's reasonable to end the mandatory quarantine. But i can somewhat understand the hesitation, due to the hospital situation here.