This video gives a good look at what’s involved in building a stone retaining wall like this:

Essential Craftsman: How to stack a boulder wall

There’s a fair amount of practical expertise, like picking specific sizes/shapes for specific areas and arranging the rocks so they won’t move later (he talks about the long-term safety implications around 12:30). It’s a very involved moment-to-moment decision making process.

Just because the robot can fit the rocks together in a way that stands up now doesn’t mean it’s done so in a way that will be safe and stable five years from now, especially with the pressure of tons of dirt behind it, probably with a building on top of that.