I live in Ireland. 10+ years ago I used to the travel to the US quite a bit (5-10 trips a year). The majority of the time I would fly via a major EU hub like AMS or CDG even if there was a direct flight from Ireland.

The reason? Other factors that were important to me!

Examples:

- cost optimisation. I could fly business within company budget rules.

- flight time. I highly valued longer flight times between Europe and the US west coast so I got a proper night's sleep when flying east-bound. Doing connections on the east coast often meant quite short transatlantic flights that arrived very early in the morning. Much harder to manage the jetlag.

- Product offering. Some of the routes from AMS and CDG had much newer planes and cabins.

- Airline alliance loyalty. Given the travel pattern it meant sense to pick an alliance that delivered some value. Irish airlines didn't have much to offer and were much more expensive for a worse end to end product.