When you have a car, but you don't have a transit pass, the cost of completing a particular trip by car is (or at least appears to be) cheaper than doing so by public transit. For example, if I have to go to the city center and back by tram, the cost for two one-way tickets is € 4,60. The cost of one liter of gasoline is somewhere below € 2,00. Public transit only becomes cheaper than going by car if you use it regularly and thus purchase monthly or yearly passes.
When you have a car, but you don't have a transit pass, the cost of completing a particular trip by car is (or at least appears to be) cheaper than doing so by public transit. For example, if I have to go to the city center and back by tram, the cost for two one-way tickets is € 4,60. The cost of one liter of gasoline is somewhere below € 2,00. Public transit only becomes cheaper than going by car if you use it regularly and thus purchase monthly or yearly passes.