Japan simply thinks they have apologized enough. They call their current situation "apology fatigue". Basically the people are tired of feeling like they have to keep apologizing for something that happend "so long ago". Any Japanese politician who would issue another formal apology would commit political suicide. They would lose the support of their own party and of a big part of the population. This isn't just in regards to South Korea but towards all of Japans neighbors.
So for now Japanese politicians who want the two countries to work together try to find wiggle room. Japanese politicians generally acknoweledge that the South Korean people went through very hard times but do so without admiting Japanese fault.