It depends on the ticket fair classification of the passengers booked in Coach on an oversold flight. They will upgrade their Metals first, and then others in order of booking class. Lastly they will put ppl in first if they are going to deny boarding to any other passengers. (This will be more so now with the new federal laws in effect)
The passengers you see on the standby list are burning miles or free upgrade vouchers, or bought the right to standby for an upgrade. DAL just has that many ppl that have miles to burn.