This is probably a repeat, but one big reason is "reason" itself. Modern education is based on the scientific method, which demands that any hypotheses must be backed up by repeatable observations. And for mortal mankind, this is mostly a good thing, because we should make our decisions based on rational observations. [I won't get into the recent trends of pseudo science which are harmful to our advancement]
People who don't believe in God think that if they can't observe miracles, then it must mean God doesn't exist. I fell into that category when I was younger.
But now my eyes have been opened. For starters, one must ask "where do the laws of physics come from?" Then, "I think, therefore I am. Where does my consciousness come from?" Our ability to think and invent ways to manipulate the laws of physics to our benefit (or sometimes harm ourselves) is not explainable unless you admit the answer to my two questions is "God".
God has already performed miracles: creation of the physical universe we can observe, and creation of life capable of observing it. The true laws of the physical universe are constant and unchanging. We (mankind) represent independence, randomness. Our actions are outside of the physical laws. God does not need to perform miracles on demand like parlor tricks. And if and when He does, then they probably usually follow the laws of physics He laid down, so the observer of those miracles would normally dismiss them. They are subtle. Great miracles, like parting the Red Sea, or the coming of Jesus Christ, are also likely. In all of time, such miracles may be common, but in our individual lifetimes, appear to be rare.
In the words of a great philosopher, "God is everywhere, God is always." And I would like to offer an additional observation: God is through all of us.