Re: you might think some generic assemblies
Wheels and drive mechanisms should be common (maybe different seals, tyres) but an electric motor is still common to both as are springs / shocks
If these were mass produced, and the mechanical requirements sufficiently understood this could be true. But neither are. Eg. one of the rovers was found to have tyre problems: one of the surfaces had lead to damage.
There are very few such vehicles built, each one is going somewhere new. Each will encounter conditions not previously experienced.
The simple approach of "make more robust" fails when mass is extremely constrained. Every gram put into a better spring, is one gram less for the science.