Good question! I think we do have some solar orbiters out there. Don’t know if they have to be 100% dedicated to other tasks or even whether they can be used as relays in the first place. Guess NASA doesn’t consider it to be that much of a problem.
I’m guessing it just not a major issue for the mars missions to idle for a couple of weeks. At least not enough of an issue to mess with relaying through other satellites. It would be different if we had a human crew there.