Playing the game really is the only solution. Just try to focus on the races with "New" badge on top of it, and you'll unlock all of the races in no time.
You see, that wasn't so hard. Someone who played the game with a strategy in mind. In order to open all the race tracks, you need to enter races track with a "New" badge. Very smart my friend!
So all this time, I thought if you "play the game" by driving around the map, the game would automatically unlock all the race tracks.
Looks like they have changed the way to open all the race tracks or open the entire map. In Horizon 3, if I can remember correctly, the strategy to open all the race tracks was to win as much as you can in the "Fan" points.