Apparently this game has a few bugged achievements and there's a good chance you would run into the same issue as I have — I had to beat the game twice only for the "Ultimate owner" to not pop up anyway.

It seems a lot of people have the same problem, i found multiple forums online talking about it but none of their recommendations worked for me

How I solved it:

1. Buy every car BEFORE going into the final race
   
2. Once you win the race, go into the car menu and select the new car
   
3. When the achievement doesn't pop, quit the game
   
4. IN A BRAND NEW PROFILE buy any car
   
5. That should do it, it did for me

This could have saved me roughly 5 hours of grinding all these race and the money for cars from scratch