With the 1,000s of different versions of religions and the different versions of those religions, an appearance by God is long overdue. There also billions on Earth who would welcome the knowledge of its existence just to confirm which version of its being, religion, messenger, prophets are right.
By not showing itself it only confuses the issue and leaves open the possibility that it's just a manmade creation. No theist really wants that, even if their religion might not be exactly right at least they would know how close they are to the truth.
Thinking about your question, I can only think of one answer. Because you would be dead.
To explain...
If God were to show himself,
1) man would die from his glory. It would be worst than asking to see the sun 1 million miles away.
2) man would die trying to capture God, like they try to capture anything they find and want to know everything about. So man's trying to capture or kill God, man would die when God swats them like flies.
3) it would make no difference to people who claim they want to see God... and this is seen from the fact that when Jesus gave proof of divine origin, people wanted to do away with him, and did so.
They killed the prophets of God, not because they did not believe, but because they did not want anyone telling them what to do.
That's the real issue, apparently... according to what we read in the Bible.