Closed-Won Is not an Option as a Status of a Newly Created Estimate

User creates an Estimate, the Status field lists the Customer statuses for the stage (Prospect).
 
These are found under Setup > Sales > Customer Statuses. If user does not have a Prospect status (Closed Won), this option would not show up on the list of statuses from the Estimate record.

User may create a custom status (Closed Won) under the Prospect stage with a probability of 100% if user wishes to see this as an option.

1. Navigate to Setup > Sales > Customer Statuses > New.
2. Enter Status name. 
3. Set Stage to Prospect.
4. Set Probability to 100%. 
5. Save.