>>42498202It has to do with their type weaknesses and resistances and both the science and symbology behind it.
Ground types are hit neutrally by Fighting, Steel and other Ground type moves while Rock is weak to them because of Rock’s “hardness.” Unlike Ground which is pliable enough to cave under those attacks while retaining it’s Ground-ness Rock will shatter into unfixable pieces forever. Similarly Ground resists Rock because unlike Steel rocks do not make very good digging tools unless shattered into tool-shaped pieces (though one could make an argument here for the easily shattered clay-made ground types). Rock’s resistance to Flying, Normal and Fire which Ground does not share can also be explained by “hardness” that Ground does not have especially when referring to the clay or sand based Ground types.
Ice is super effective to Ground but not Rock because when water in the earth freezes it can break the ground as it expands leaving fissures as the weather heats back up. See the craters that form when permafrost melts for an example. This is supported by two weaknesses shared with Rock— Grass and Water— Who are capable of consuming and eroding Ground and Rock equally in similarly scientific ways while Rock is fine with Ice. Ground’s immunity to Electric however seems mainly based off of the “grounding” pun but considering some rocks and gemstones are capable of holding an electrical charge there is some basis in reality there.
Tl;dr: Rock types are so hard that they can be shattered irreparably but resist a certain level of heat and pressure. Ground types are malleable and more susceptible to erosion but cannot hold any electrical charge due to both puns and science.
