I think maybe the fact that it seems so natural is actually part of the reason. Ground is, let’s face it, a poorly thought out mess of a type. Pokémon can be assigned to the Ground type because they have powers related to earth and soil, or because they happen to live on the ground, or sometimes, it seems, just because they’re generally tough and resilient. It’s a really vague set of traits, most of which also apply to pure Grass-types. If most of the things that define Ground are also essentially inherent to Grass anyway, there’s never any need to add Ground to a Grass Pokémon, unless you come up against something like Torterra who’s associated with the earth in a much more elemental sense than most Ground Pokémon.