Because it's part of the stuff that constitutes who you are. The knowledge of x is a property that belongs to you; it's something you necessarily possess if you possess it. According to your argument, God's characteristic of knowing what we will do is caused by us which means he has a characteristic that is caused (by us no less). So if God is the constituent of various characteristics (like all powerful, all knowing etc) then there's a part of him that is caused. o.o
It's a classic problem ! Don't blame me I didn't invent it
If you want your head to spin, read maimonedes' answer to this LOL.
It's no more a "problem" than that pathetic "problem of evil". It's amazing how many great minds will work themselves into a tizzy over absolutely nothing. If you can't tell the difference between an entity itself and a certain trait the entity just so happens to possess, it's your own problem, and I don't know if I can help you with it. The only "defining" characteristics of God, insofar as there really are any (that we could ever understand), is that He is the creator and ruler of the universe--as any dictionary in the world will attest. Everything else is just certain qualities that God has, and which if gone would not mean an absence of God but instead merely a different kind of God.