Why does he want to test us?
I can't say that the ONLY purpose is just to test us but it's just one of the reasons which Allah told us about. The story goes back to what God said in the Quran, when the angels--who commit no sin and worship and obey God without a question for not having any free will at all--asked God, why would he create something like a mere human being who, unlike angels, is capable of doing all sorts of evil things. But God wanted man for something unique which pure angels currently not capable of, because man, will be able to use his mind to do great things as well beside evil things. He will be able to *choose* to do these good things, not to be able to do it by nature like angels. And God knows best.
And [mention, O Muhammad], when your Lord said to the angels, "Indeed, I will make upon the earth a successive authority." They said, "Will You place upon it one who causes corruption therein and sheds blood, while we declare Your praise and sanctify You?" Allah said, "Indeed, I know that which you do not know." (Quran 2:30).
Can you explain to me how having faith in something without proof is a virtue? Wouldn't this test just show who was willing to believe wild claims without evidence? I fail to see the purpose behind this "test".
Well, each one of us see it as the right thing to do, or the path, way of life, more than just virtue. Even non-believers do virtues but will the road they take in their whole life is the right one?
The test, by the way, although i tried to simplify it but it's more than mere believing in him withouth seeing him. You need evidence to believe, and each of us has his own evidence of God's existance. Wait! i don't want to hijack this thread and turn it into another typical creation vs. evolution thread!
So to the question in the OP, why can't God reveal himself? he certainly can, but one might ask, why would he?! there are plenty of answers out there by everyone.
God bless you all.