Is it normal for master's students to be sidelined, compared to PhD students?
No idea about anything specific to Korea, and haven't personally experienced this, but yeah, it seems pretty normal to me. In my field in the US it is similarly most common for people to be admitted only to PhD programs, with a masters degree as an option for people who fail to/decide not to continue.
From the perspective of your coworkers, you've decided to quit the lab. You don't mention why, and it definitely might be the right decision for you, but for people around you it might look like you don't value the kind of work the lab is doing, you're just going through the motions to get the master's degree and move on. You also might be perceived as a bit of a burden: you're opting to skip the years in which you would be likely to be most productive and most helpful to others as a senior graduate student in the lab, and only staying for your first steps into research. Ordinarily, if your goal is to &quot;study more and learn how to be a researcher&quot;, the best way to do that would be to complete a PhD program, not to quit with a master's.
I think it's unlikely you'll change this attitude much, so I think you probably have to just embrace your decision, put your head down and get through it. I don't think that's really fair, by any means, and is especially unfair if you're working just as hard as everyone else while you are there, but I can also understand why your coworkers might not feel like you're a full contributor.