I don't speak Swedish natively, but I think it's because "will be" translates better as "kommer att vara" or "ska vara". "Blir" emphasizes the change, so it's more like "become"/"is becoming". Also note that "will be" is in the future tense, while the Swedish sentence here is in the present tense
It's worth to mention that "God natt" is actually not used as a greeting or farewell phrase in Swedish as it is in English. "God natt" is said just before going to bed really. It's almost a synonym to "sov gott" (sleep tight), but you could say it to someone who were still staying awake as you went to bed which you wouldn't do with "sov gott".