Why don't preachers provide verification of miracles done through them?

For many years now, I have listened to charismatic preachers retell how miracles were done through them. I have read books that retell of prophecy, healings, words of knowledge been done.
But I have never heard or seen them provide any proof they are saying is true.
Especially when we read of so many pastors and mission agencies caught in financial and sex scandals and lying to cover up.

How do we know they are being truthful?

Even Jesus, who is the Son of God followed this biblical principle.

“If I were to testify on my own behalf, my testimony would not be valid. 32 But someone else is also testifying about me, and I assure you that everything he says about me is true. 33 In fact, you sent investigators to listen to John the Baptist, and his testimony about me was true. 34 Of course, I have no need of human witnesses, but I say these things so you might be saved. 35 John was like a burning and shining lamp, and you were excited for a while about his message. 36 But I have a greater witness than John—my teachings and my miracles. The Father gave me these works to accomplish, and they prove that he sent me. 37 And the Father who sent me has testified about me himself. You have never heard his voice or seen him face to face, 38 and you do not have his message in your hearts, because you do not believe me—the one he sent to you. (John 5)

At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. 11And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.” (Mark 1)

There were 2 supernatural witnesses to testify that Jesus is the Son of God. God the Father, and The Holy Spirit. And there was one human witness testifying about Jesus - John the Baptist.

Thus the accounts of Jesus life (gospels) were written - not by one person, but by 4 authors.

A few specific instances where Jesus provided verification:

1) "...and a man with a shriveled hand was there...  Looking for a reason to bring charges against Jesus, they (Pharisees) asked him, “Is it lawful to heal on the Sabbath?” Then he said to the man, “Stretch out your hand.” So he stretched it out and it was completely restored, just as sound as the other.". (Matthew 12)

There were witnesses who could record what had happened.

2) "Some men came carrying a paralyzed man on a mat and tried to take him into the house to lay him before Jesus. Which is easier: to say, ‘Your sins are forgiven,’ or to say, ‘Get up and walk’? 24But I want you to know that the Son of Man has authority on earth to forgive sins.' So he said to the paralyzed man, 'I tell you, get up, take your mat and go home.' 25Immediately he stood up in front of them, took what he had been lying on and went home praising God. 26Everyone was amazed and gave praise to God. They were filled with awe and said, 'We have seen remarkable things today.'" (Luke 5)

Jesus was providing verification to all. It was not an invisible healing.