I found about 2.1M contacts by searching contacts that don't have any created date. They don't seem to have any other relevant data other than first and last name. Can anyone tell me more information about these?

Try this query to see the first hundred.
Select *
From civicrm_contact
Where
is_deleted=0
and contact_type='Individual'
and ISNULL(created_date)
Limit 100;