I am no expert, but I think it is very likely most of these craters are from very early life of the planet, before there was life on Earth. Orbit of Mercury is very well cleaned of any other bodies and must have been like this for billions of years.
Because there is no erosion mechanism on Mercury (except for crumbling due to temperature changes) these craters were preserved perfectly.
Because there is no erosion mechanism on Mercury (except for crumbling due to temperature changes) these craters were preserved perfectly.