It's just a matter of safety margins and probability. Traditional powerplants break too, and with enough cars the probability that enough of them wouldn't be available at once is smaller than the probability that enough powerplants break at once.
There is so much work and planning that goes on between the utilities and regulatory agencies to ensure we almost always have reliable electrical supply. Traditional plants have planned outages for regular maintenance, and they are constantly working to ensure there are enough backup systems and alternate routes to ensure there are not outages even during maintenance windows. The amount of cars and the amount of extra grid you would have to build to completely replace these traditional plants would be prohibitively expensive. It's not just a matter of plugging a few million cars into homes.
It's just a matter of safety margins and probability. Traditional powerplants break too, and with enough cars the probability that enough of them wouldn't be available at once is smaller than the probability that enough powerplants break at once.