The last ocean liner to enter service was the Queen Mary 2, which completed construction in 2003.
Cruise operators don't build ocean liners anymore because of the popularity of modern cruise ships. Modern cruise ships can sail worldwide and have a wide range of amenities and activities you won't find on an ocean liner.
Another reason for the decline in popularity is the speed and safety of air travel. Although some ocean liners served as cruise ships, their primary purpose has always been transportation. But, with the lower cost of air travel, it doesn't make much sense to travel by ocean liner.