They're very indiscriminate about who they kill, so civilians often catch the brunt of them
They can last for a very long time, and are capable of killing well after being put in place - there are instances of land mines placed during the Second World War causing injuries and deaths in recent years.
Just because they're put in one spot, doesn't necessarily mean they'll remain there. Soil erodes and runs off, and land mines are just as capable of doing the same, and there is no way to account for them at that point unless they're discovered by chance, or they detonate.