> I believe most newer cars do this and there are good reasons for it.

That's an incredibly vague and dismissive statement to support lazy engineering.

If the intent is something like preventing CO2 build up in the cabin then they could periodically open the mixing value when the car is set to recirculation to refresh the cabin air.

It doesn't have to be a binary control. I've owned cars with manual mixing valves to let you control the amount of outside air gets circulated into the cabin.