"The Pharisees came and began to argue with him, seeking from him a sign from heaven to test him. And he sighed deeply in his spirit and said, “Why does this generation seek a sign? Truly, I say to you, no sign will be given to this generation.” And he left them, got into the boat again, and went to the other side" -- Mark 8:11–13God doesn't just want you to know he exists. He wants a relationship. Showing off his power while demanding your obedience isn't the way to go about that and he knows it.
I mean, yeah, maybe if God did something amazing like appear as a giant and speak to you personally telling you that he was there and he wanted what was best for you, you would probably be convinced that he was real, but would that display really make you want a relationship of any kind? Just knowing he was real is enough for you to change your life and follow his lead?
I'm not convinced.
It's also kind of strange that sceptics think this personal revelation would be convincing, when they so often dismiss the personal revelations of others, and many even suggest that the disciples of Jesus who really saw their old friend risen from the dead were simply hallucinating.
You're sure that if you saw a 100 foot God speaking to you, you would believe and not tell yourself it was a dream?
This is why God doesn't bother with miracles like these. They're not worth anything.