Why is prophecy given instead of only fair warnings?

To show off, obviously. I mean, any idiot can guess that things might go wrong if someone doesn't do something. It takes some serious God-Power to tell people exactly how things are going to go wrong and then make them go wrong that way despite everyone's best efforts. That's just impressive. It's like saying what part of the offensive line you're going to blitz and then blitzing it and plowing through the entire other team and sacking their QB, his coach and half of his extended family. I mean, damn. You gotta respect that.

That, and some prophecy is good news, so the whole fair warning thing wouldn't really make sense.