When people have dreams regarding the future that come true, where does this come from?
There are several problems here. In times past, God did speak to people at times through dreams. "God, who at various times and in various ways spoke in time past to the fathers by the prophets" (Hebrews 1:1). However, there were many people having many dreams which were not communications from God. What we conclude from reading the Old Testament is that such dreams from God were rare events, which is what made them noteworthy.
But the Book of Hebrews writer's point was that such means of communication had come to an end. "God, who at various times and in various ways spoke in time past to the fathers by the prophets, has in these last days spoken to us by His Son, whom He has appointed heir of all things, through whom also He made the worlds" (Hebrews 1:1-2). God used to talk to a variety of people in a variety of ways. Now He communicates through Christ, and Christ communicated to us through the apostles and prophets of the first century.
But going back to the first point, what about people who dreamed of a future event and it came true? It still has several problems if you want to claim it is from God. First, dreams are, at the root, you watching your brain organize what you experienced so that you can recall the information at a later point. Since people are able to look at facts and draw conclusions about future possibilities, it would not be noteworthy that such conclusions can show up during dreams. Second, people tend to be selective in what they take note of. You notice when your dream happened to get a future event right, but how many times did you dream of a future event that you got wrong? I suspect the number is far higher, but you don't pay attention to the failures. There was a famous psychic back in the '60s who predicted the death of John F. Kennedy. She sky-rocketed to fame, and even though she constantly failed to predict other events, she remained sought after because she got it right once. "If there arises among you a prophet or a dreamer of dreams, and he gives you a sign or a wonder, and the sign or the wonder comes to pass, of which he spoke to you, saying, 'Let us go after other gods' -which you have not known-'and let us serve them,' you shall not listen to the words of that prophet or that dreamer of dreams, for the LORD your God is testing you to know whether you love the LORD your God with all your heart and with all your soul" (Deuteronomy 13:1-3). Even in the days of God speaking in dreams, there were false prophets who sometimes got the future right.
Finally, there is also the problem that people have in altering their memories. If a dream came close to being right, but a few details were wrong, people will take note of what they got right and forget the mistakes. Does that mean they predicted the future? I would argue, "no."
The problem is the claim of being able to predict is always after the fact. Memory is not an accurate record, it can change. Memory can't be doubled checked by another person to be verified. I've challenged a few people to write down their dreams when they think they are foreseeing the future. It doesn't take long for them to give up because they soon realize how often they don't really see the future. Their view of the future is no better than their waking guesses.
"I have heard what the prophets have said who prophesy lies in My name, saying, 'I have dreamed, I have dreamed!' How long will this be in the heart of the prophets who prophesy lies? Indeed they are prophets of the deceit of their own heart" (Jeremiah 23:25-26).