Cars are not simply unlocked and put into your garage they are unleashed into the world for you to find and when you find them its time to race.
I often found that the cars I was looking for would drive past me in the opposite direction, forcing me into a 180 handbrake turn. That chase is so much more enthralling because it started with an exciting maneuver and not a yes response to a pop up menu.
I think keeping the player in the world and focused on driving as much as possible is so important for immersion. Perhaps the Burnout Paradise world is not as rich as some story based games, but immersion is about being lost in the moment. Staying in the car is so important to that feeling, something supported by Jordan Mechner,