This is already common in the satellite world. Companies design & build satellite platforms (usually called "buses" for sats) that satellite developers build on top of. Usually provides the core physical structure, plus attachments for payloads or sensors; some more advanced ones also include stuff like avionics, power distribution etc. Wikipedia has some interesting examples [0].
Photon mentioned in a sibling comment builds on that idea by providing a bus with engines to make it into a simple spacecraft. I don't know of any off-the-shelf rover platforms yet but can absolutely see the same thing happening when there's enough demand / cost of access to places worth roving around is cheaper.