That is probably because you look at it from too narrow a position. Yes, the public transport inside Oslo may be brilliant. That is very good for transporting people around who lives inside Oslo, but chances are that a lot of people cannot afford or want to live there. For probably each kilometer extra you move away from the city center, the difficulty using public transport increases exponentially. Once you get to 50-100km away from the city center, then your options look very bleak.
I live in Copenhagen myself, but I grew up 100km outside of Copenhagen. Using a car to go back and forth from there to Copenhagen takes approximately 1 hour each way. Using public transportation all the way takes closer to 2 hours. Using a combination takes 1h 30min on a good day.
Some people move out of cities because they want the countryside. But other people are forced to do so because of prices. Banning cars from cities seems to only think of people who lives inside it and not how difficult it will be for a lot of people already "taxed" with hour long commutes because of stupidly expensive housing combined with centralization of workplaces.