Difference between db_owner and having all other database roles except the 2 containing the deny keyword
As per the Docs article, db_owner can perform functions that no other role or combination of roles provides, including but not limited to:


Adding users to fixed-database roles
Dropping the database
The CONTROL DATABASE permission


There is a diagram on the linked article that shows the fixed-role permissions for each role. Any permission not listed in that diagram is granted to the db_owner role only by default.