Mat 16:4 A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." Jesus then left them and went away.The proof offered by God was the resurrection of Messiah. It was not a proof welcomed in His day, and still appeals little. But it was the sign that was given. As a matter of history, you might look into it and do your due diligence. Did Jesus provide the proof He claimed as to who He was?
Beyond this, there is this same proof in many lives... of supernatural resurrection power. Many have studied out the resurrection of Jesus for many years in a scholarly fashion, coming to a knowledge of God (Theology) this way. Each must be honest before God as to their own search for truth. But in my case, I saw a resurrection inside of me: full of joy and new-instincts and peace and righteousness... and suffice it to say that I was an evil person before this "new birth". Many have seen this same miracle in others, and have believed that they too can change, and know God as well.
You seem sure that if you were to see an external miracle you would believe in God. I think this is naive. First, I have seen many such, and it was not as if these signs changed me. The problem with the supernatural is that it is, by ingrained experience, impossible. Thus, if you experience a miracle, it quickly fades from view like some sort of dream. If you talk about it with people, then it almost seems as if you are lying, even though you are not; for you can sense in those listening how improbable what you are saying is. Martin Buber explored this in some depth: this insult to reality that comes from description. This is why I think Jesus nearly always told people "not to tell" of the miracles He did. He did them out of compassion and love, not for news. His sign was His own life, offered up, and then made eternally alive again. "The sign of Jonah".
Also consider the Israelites. They saw daily miracles, and they were fickle and unbelieving. Think about it: a repeated miracle become "normal". I suppose the sun rising every day could be considered a miracle of sorts, but we take it for granted, just as the Israelites did the manna after but a few weeks. I have seen this tendency myself, and in myself. Some rather radical transformations God has worked in me I have taken as "normal" after a while, when it is clearly not so.
I think God has proven the assertion that "external miracles are what is needed to change people" is wrong in history and experience. If signs could save us, then I am sure He would give us more. But we would make a "law" out of them, and take them for granted, or our minds would prove fickle and our stories harder to believe, even by us. Miracles can be glorious in the moment, but in the long run "a wicked and adulterous progeny seeks a sign", Jesus said.
I have seen many miracles. To tell you them would cheapen the experiences, and in any case they are "unbelievable" by their nature, and thus for me to even consider them many years hence makes me wonder. While I thank God for all He has done in my life in the past, right now there is no dramatic miracle. I must resist the sin instinct within and follow Christ in identification with His death and burial and resurrection so as to participate in His life. How can a supernatural manifestation seen years ago help me now? If anything, it makes me mad. Of course, I was but a spiritual kid then, and God was coddling me, perhaps. But now the training wheels are off and instead of three minutes of "waiting" for resurrection power... it is three hours. Perhaps I will grow up to three days (metaphorically speaking, of course) as Jesus did. But for now, my faith is not in past (irrational, unbelievable) miracles, but in my trust in the character of my God whom I know.
2Tim 1:12 (NIV) ...I know whom I have believed, and am convinced that he is able to guard what I have entrusted to him for that day.Knowing God is the goal, not miracles or signs. If we know Him, then "signs will follow" as the Scripture says (Mr 16:17). They do not lead, they follow.
Believe in Him to the extent you can, honestly, and begin to "seek Him while He can be found by you." (Is 55:6)