Why are tickets from CDG to ATL so expensive?
 

ATL is a fortress hub for Delta, and by extension, Skyteam. So the DL/AF/KLM can charge whatever they want and take the profits. I think DL has something like 80% of the slots at ATL.

It was the same for CLT (Charlotte) and AA being a former US Airways hub. AA owns 90% of the slots and makes huge profits there. Business travellers need the schedules and the companies pay for it.

It is also a joint venture AF/KL/DL on transatlantic flights.

That means AF cannot sell ticket lower than the price DL suggests, even all segments are on AF's metal. They have an agreement on the prices and DL's revenue management department thinks they can still feed the plane with last-minute traffic, they will never put down the price, on both AF and DL flights.

Because there is no non-Skyteam competition on the route offering direct non-stop service.

And because the flights fill up with connecting pax bound for plenty of other destinations offered by Delta in the US, they have absolutely no incentive to offer a "competitive" price on such flights.

Conversely, that is why ORD and DFW which are hubs for DL's competition are so cheap to fly to.