Because the "secret rapture" doctrine is not biblical and would not even be in anyones minds if Cyrus Scofield didnt include it in his margin notes 100+ years ago.

As for the danger of such a false doctrine, it leads people to believe they will skip the mark of the beast crisis and the time of Jacobs trouble.

Scripture teaches Christ does not return until the 7th last plague is in effect. Not before. And not in secret.