Our cities aren't densely built up, except for New York. The actual urban area of most cities generally has far fewer people than the suburban metroplex surrounding it. 6.5km is literally larger than all of downtown Dallas, depending on how you define downtown.
Even our cities are designed for car travel, so unless it's rush hour you're still faster by car. Unless there's a concert or other event happening, it doesn't take nearly 20 minutes to traverse downtown Dallas in a car.