Can a U.S. citizen file a fiancé(e) visa if they've never met in person?

No—K-1 visa rules require that the couple must have met in person within the last 2 years.

Only rare waivers are allowed for cultural or religious reasons

Failure to meet the requirement leads to immediate rejection of the petition

Honestly, from what I know, you can’t file a fiancé(e) visa without meeting in person within the last two years