Well, the truth is that all (or almost all) airlines which pre-assign seats generally leave a good number of seats open until the day of the flight in order to accomodate elites or full-fare passengers booking at the last minute. CO
especially with its EliteAccess guarantees of a window or aisle seat for full-fare passengers would want to leave some desirable seats open. Certain seats (for example, bulkheads near the front) are also held for passengers who might need a bit of assistance.
If you were a OnePass Elite you'd probably be able to reserve a seat but since you're not, you can't.