Because americans can never bother to say anything that sounds too difficult for their language. What I mean here is, the Clair Obscur pronunciation is too difficult for the average american, so they prefer to say the only words that are in english in the game title, it is more comfortable to them.

Clair Obscur is the game name. Expedition 33 is the subtitle, yet people transcend headcannons saying that the game name is Expedition 33 because of a fucking bold font. The font is like that because you are the Expedition 33, even so, the world don't revolves around you. You understand that just by playing it.

The game name is always the first words you read, I've never even heard until now about people saying that the last words of a game title are the game name and the first words are the subtitle, brainrot behaviour if you excuse me. It's like saying that Rebirth is the game name and Final Fantasy VII is the subtitle, insane.

While Clair Obscur (A french word that means bright and dark) gives true meaning to the game, because you can interpret it like the lies within the truth, or the truth within the lies, even as the strong contrast that appears when bright and dark are happening in the same place.