>>43867105It's purely for Type effectiveness reasons.
While Ground Types are usually just burrowing or arid-dwelling animals, Rocks Types are usually actually MADE of rock. So physical differences need to be considered. Rocks are physically hard, heavy and dense whereas soil is loose and behaves more fluidly and absorbs impacts differently. This is reflected in the fact that Rock Types are generally slow with high defense but Ground Types generally have high attack stat.
Rock differs from Ground in these key ways (these would be conflicting/wouldn't make much sense if they were a combined type)
>resists normal>weak to fighting>smashes ice like glass>crushes bug's exoskeletonand probably biggest one
>rocks knock birds out of the sky but if they can fly they should also be immune to an earthquakeAlso Rock is representative of ancient fossils as well as gemstones which starts to maybe be a bit much if all of those concepts were widely covered by just one Type. A Type that covers too broad an area thematically risks not having any real identity.
An idea I kind of liked the other day was combining Rock and Steel into one Type, maybe called Mineral. It's worth thinking about because both Types are trying to be heavy and defensive but Steel clearly outclasses Rock and makes Rock kinda pointless. Steel also doesn't have much of an 'identity' beyond just being metal-clad animals.
