Eh, Koreans have a pretty high rate of smoking (this site claims >40% of adult men), so you're probably just getting sampling bias if most you know don't smoke. My SO's friends are all anti-smoking for religious reasons, but when we visited Korea, a lot of people smoked. A lot of people in Asia smoke, it's nuts.

But yeah, Koreans tend to stick with other Koreans. My in-laws are quite racist against non-Koreans (I get a pass), and while the younger generations are better about it, they still tend to stick with other Koreans. I had a lot of international friends growing up (Vietnamese, Japanese, Taiwanese, etc), but none of them were Korean, despite having a pretty big Korean population where I grew up. I had a good working relationship with one in college when I was dating my SO (he was super good at StarCraft, that's not just a meme), and I helped him edit his essays.