Get Me a Round Rock
It’s like people will intentionally go out of their way to give their homes the least support possible. Home builders use concrete as foundations for a reason: it’s sturdy, it’s long-lasting, and most importantly, it’s shapeable while still wet. This is crucial since you can form a flat surface for a wooden column to stand on.
Instead of doing the reasonable thing, this person decided to work with what they had. There just so happened to be a boulder in the right spot to serve as a foundation. Why go through the trouble of making a foundation when you have one right there?