They don't have to be though. Spirit and Opportunity entire project budget (including design, building, launching, and operating for 90 days) was 820M. Even with the mission extensions the total cost is < 1B. And a lot of things have changed since then. Launches are cheaper, tech has improved and some recent missions have proven that even CotS hardware can exceed expectations (see the helicopter).
I would love to see NASA do something like the CLPS but for Mars. They could pay for launch services (which are way cheaper now with F9 / NG), and help with EDL (using the same parachute + airbags thing that has worked before), and leave the rover parts to 3rd parties.
We could have universities join the competition, building the rovers, exploring CotS stuff, autonomous driving and so on. Lower stakes than the decadal big rovers (Curiosity & Perseverance), but also cool and useful to train the next generations of students. Hell, I bet even companies could enter the race, with Toyota / Tesla / whoever else supporting this effort.