From the government perspective, the reason they are discouraging driving as a pastime is the risk of an accident. They know we need to do it for our mental health but even if it is a single car incident, emergency responders will be required at the scene. The risk is low if only a few cars are out for a "Sunday drive" but if the government did not discourage it, then thousands of cars would be on the road for that Sunday drive, everyday. Accident rates go up and more responders required. People without the virus could be exposed to others with it, who don't know they have it - think paramedics and cops. The reverse is also a fact.
Before retiring in January, this was my job for 18 years. I managed a consulting company that helped large enterprises and government agencies prepare for the unexpected with business continuity plans (BCP) and emergency plans. One type of BCP is a pandemic plan that is based on a worst case scenario. We did a lot of that work for the Feds, Ontario and some large enterprises in 2004-2010 after SARS. Starting in 2011 the urgency diminished and the plans were not updated and exercised. We are seeing the result.