Why would God NEED to predestine those He already perfectly knew would come?
He didn't. His foreknowledge of our free will decisions is all there is to His predestination.
That is a very, very good question, for which I do not have an answer. The only thing I can think of is that when we try to fully understand the sovereign, infinite mind of God with a finite mind, we will always come up on the short end of the stick.