They’re not. You’re referring to religious propaganda films which are made with the express purpose of advancing a narrow evangelical ideology, one that is basically antithetical to good art. Their audience is perfectly happy consuming this schlock, so they don’t bother changing the formula.

If you would like to enjoy one of the “few good ones” (lol), I’d look into Tarkovsky, Bresson, Scorsese’s spiritual output (Last Temptation of Christ, Silence, Kundun, Irishman to an extent), Dreyer, Bergman’s Winter Light, Lourdes by Jessica Hausner, or Uncle Boonmee Who Can Recall His Past Lives. Also, read Paul Schaeder’s Transcendental Style in Cinema, which is a bit outdated in terms of criticism, but has many great things to say about the deep connection of spirituality and art film.

I know what your question is getting at, but it’s baffling that the question “why are religious movies so bad?” could possibly be upvoted here.