A) land mines are banned in most countries for that reason
B) mines don't move by themself, but stay where you deployed them
A moving autonomous killer drone has a potentially bigger effect if unleashed on the wrong area. Besides, war is usually fought in civilian areas. If you send the drone to the frontline, but its GPS is jammed, it might move somewhere close by.
A) Most countries also have laws/treaties banning invasions. But when those happen to them anyway, most countries start ignoring the bans on landmines. Which are quick/cheap/easy to produce, even with century-old tech.
B) https://en.wikipedia.org/wiki/Naval_mine Not that anybody's doing Environmental Assessments on land, to determine whether the mines might be washed downstream after a heavy rain, or moved downslope by a landslide, or ...
The problem you describe sounds very much like poorly-directed mortar/artillery fire & strategic bombing - which have been killing civilians at scale since at least WWI.
B) mines don't move by themself, but stay where you deployed them
A moving autonomous killer drone has a potentially bigger effect if unleashed on the wrong area. Besides, war is usually fought in civilian areas. If you send the drone to the frontline, but its GPS is jammed, it might move somewhere close by.