For most of the contending teams 12 players is not the optimal number. The salary cap rewards taking that $70,000 or so you might spend on the 12th player, and instead upgrading players in the rotation, say $40,000 more on your 6th player and $30,000 on your 7th. You get more value by spending your money on rotation players and filling out the rest of the roster with near minimum players.
Teams near the bottom are different, they frequently can't spend their large cap space effectively, often overpaying for players to encourage them to go to a losing team. Usually they still have cap space for a 12th player and hope they can find a diamond in the rough that turns out to be a good value.
I think the top contenders also prefer to fill out the tail end of their roster with known vet minimum players, than rookie prospects. Expansion will help but the number of teams and the size of the maximum roster are still very small.