I'm more concerned with your apparent displeasure towards Christians. Have friendly discussions and even debates, but when one party starts using an accusative tone, it's never a pretty sight.
My answer to your question is, what is the use of such a thing as Faith if God so easily shows himself to everyone? And how about what will happen exactly if He chooses to do that (show Himself to humans), humans wouldn't be able to stand in his presence because he is too holy (yes I do know I'm sounding a bit cheesy) and righteous for plain sinners to bear.
And perhaps you haven't been exposed to enough sources. There are several testimonies which display true miracles, in that the people who went through them were not Christians before, nor did they believed in God. If someone is willing to even be slightly broad-minded, he would definitely see these testimonies for what they truly are, instead of insisting that they are bullocks uttered by pseudo saints who chant and pray Amen.
Honestly, if you have already closed your mind to such possibilities, then what's the point of this question? No matter what Christians say, it wouldn't register and you would think of a million more ways to counter it. This isn't exactly a platform for you to come vent your frustrations at Christians and the Christian faith.