I've lived in 3 different apartments in Korea, of those 3 all of the doors had key locks. Some apartments have doors that you just need to put in passwords. But I've heard some bad stories with the batteries dying and the doors finding themselves left open, which is an unpleasant surprise when the owner returns.
I've had my own unpleasant experiences with key door locks. At my first place, the deadbolt style lock broke when I tried to open it. It broke in the locked position and my windows had bars on them. Luckily, being the artsy person that I am, I had some tools around. All I needed was a screw driver to remove the lock and be free of my apartment. And then getting a replacement deadbolt set (I installed it myself) was under 20,000 won.
I didn't have any problems with my second apartment, my landlords and school didn't seem to care enough to snoop around my apartment when I wasn't there, unlike some of my friends' situations. When schools provide you with apartments, they have a set of keys, along with your landlord and sometimes they may feel it's unnecessary to let you know when they're letting themselves into your apartment. Or it may just seem easier to play fast and loose with your privacy because of the language barrier, I don't know.
Either way, yesterday I happened to notice this:
Especially after my school almost did the same thing. My handler had said gas people were visiting, they would just go in if we weren't there and also tagged on "don't worry about safety ;)" which only made it seem worse. Seriously, why is adding a winking face to that sentence a good idea? I very vocally expressed that it wasn't ok unless I was there. They showed up an hour early from our set time, woke me up from a nap and I made it to the door just as they were getting the keys out to let themselves in. I was unimpressed then. That situation combined with the new one = I've had enough. And I'd like to add on, I've only been here for two months.
Today I went to the nearest 도장 / dojang I could find. Dojang means stamp or seal, but they generally have key and lock services as well. The woman tried to sell me an electronic one for about 100,000 won, but I opted for the cheap deadbolt and with my minimal Korean, told her that I needed to have it installed. I biked home, and about 5 minutes later she showed up with tools and my new lock.