In an ideal world, yes, but that's not always possible. Spouses get new jobs, living situations change, medical issues come up . . . for some reason, she wants to work with a friend in another district, she wants to move into a district where she knows a couple people will retire soon and leave openings, a better job came available for her, and that better job might not come along again for years.
You wouldn't complain about people in other jobs leaving. It's wrong to hold teachers to a different standard. If she's new, she's not all that entrenched in your child's education. I'm even less inclined to have sympathy in this situation.
Neighboring districts aren't always the same -- I would not teach in the county right next to me. And it may be more convenient for her, or it may be that her children will eventually attend those schools (that's a very big reason for teachers to move). Let's think logically instead of being upset: This teacher had eight weeks /2 months to get to know these kids; yet the new teacher will require "months"? Having taught for more than two decades, I can tell you that it takes about a week and a half to get a handle on my new classes in the fall. At the week and a half point, I know all the names (for 90-something students), have picked out the slackards, have picked up on those who need extra help. If you get a straight-out-of-college teacher, it might take him or her longer, but you're giving lots of credit to a teacher whom you barely knew and are setting yourself up to dislike the replacement.
As for sick parents, new job, etc., let me point out the key words: "That I know of". You'll probably never know just what facilitated this move.