Yes, you're right. There are a lot of South Korean players who play for Japanese teams. I know both countries are geographically close (it takes an hour by airplane only) but both are having a different which is also a barrier. But you have to know that Korean players are willingly learning Japanese so they can interact with other players as well.
In South Korean scene, the competitiveness is really thick and most of them are looking for more opportunities in my opinion. I don't really think about economic stuff because I have no idea about it at all. In Japanese scene, the competitiveness isn't as toxic as the other regions because Japan's environment is really friendly. Maybe that's the very first reason why Korean players are comfortable playing in Japanese scene. You also can find that Korean players tend to stay in Japanese team, learn the language, and even move to Japan (I'm talking about Seoldam, he's resting from the competition right now but he moves to Japan recently). Talking about Valorant as well, Japanese supporters are insanely huge and supportive so perhaps it also affects their economics and stuffs as well.