I think it's because Melissa pays her quadruple the times other maids get plus a big bonus pay when she does something like researching and others outside her profession. It may also have something to do with her loyalty to Melissa since Melissa was the only one who complies with her and is actually kind to her even after knowing that she's materialistic and was a daughter of a fallen baron(?) she was still treating her like a friend of some sort ╮(￣▽￣)╭
I hope the latter is the reason. I sooo want Yona to be faithful, like in actually caring for Melissa. She is so badass.
Yona just hit the jackpot of weath. Why does she stay a maid?