John is bearing witness to things he saw, because God is uncovering much before it happens.
One had to remember that the Jews would not believe unless they saw signs and wonders.
Jesus said to him, “Unless you people see signs and wonders, you will never believe.” John 4:48
There will be a lot of signs and wonders performed by false Christ's and false prophets. So I'm going to show you what the signs will look like. Be on guard. You will see these ahead of time.
For false christs and false prophets will arise and perform great signs and wonders, so as to lead astray, if possible, even the elect. Matthew. 24:24
The coming of the lawless one is by the activity of Satan with all power and false signs and wonders,
They pay attention too signs and wonders. They love to hear about signs and wonders.
and the crowds with one accord paid attention to what was being said by Philip when they heard him and saw the signs that he did. Acts 8:6
For they are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty. Rev. 6:14
And I will show wonders in the heavens above and signs on the earth below, blood, and fire, and vapor of smoke. Acts 2:19
And the beast was captured, and with it the false prophet who in its presence had done the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. Rev. 19:20
Notice how quiet the assembly got when Paul started talking about signs and wonders.
And all the assembly fell silent, and they listened to Barnabas and Paul as they related what signs and wonders God had done through them among the Gentiles. Acts 15:12
Israel always loved signs and wonders from the day they were brought forth. Perhaps that's why they went after a Gentile gods so often because of some of the signs and wonders that were performed by the false gods as well.
Then some of the scribes and Pharisees answered him, saying, “Teacher, we wish to see a sign from you.” Mathew 12:38
An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of Jonah.”
His disciples believed in him because of the signs he did.
This, the first of his signs, Jesus did at Cana in Galilee, and manifested his glory. And his disciples believed in him. John 12:11
The angel is showing John what to look out for when people see these signs, even from heaven.
13And the second beast performed great signs to cause even fire from heaven to come down to earth in the presence of the people. 14Because of the signs it was given to perform on behalf of the first beast, it deceived those who dwell on the earth, telling them to make an image to the beast that had been wounded by the sword and yet had lived. Revelation 13:13
Signs and wonders are things that can be seen and therefore deceive.
“If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, and the sign or wonder that he tells you comes to pass, and if he says, ‘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. Deuteronomy 3:1
it performs great signs, even making fire come down from heaven to earth in front of people, and by the signs that it is allowed to work in the presence of the beast it deceives those who dwell on earth, Rev. 13:13
There are so many more scriptures I could quote but enough said.
In answer to your question then, "why does he say what he saw" instead of what he heard regarding the word of God.
Perhaps it is because they believe signs and wonders above what they hear.
Therefore signs and wonders are given and shown what will happen before they do.
Hope is seen first and then a voice is heard.
the first heaven and earth had passed away, and the sea was no more. 2I saw the holy city, the new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.
3And I heard a loud voice from the throne saying:
“Behold, the dwelling place of God is with man,
and He will dwell with them.
They will be His people,
and God Himself will be with them as their God. Rev. 21:3
It's easier to see things in one's mind, that give one hope.
Look at what is shown to inspire hope, to endure the sufferings many will go through.
Then the angel showed me a river of the water of life, as clear as crystal, flowing from the throne of God and of the Lamb 2down the middle of the main street of the city. On either side of the river stood a tree of life, bearing twelve kinds of fruit and yielding a fresh crop for each month. And the leaves of the tree are for the healing of the nations.
3No longer will there be any curse. The throne of God and of the Lamb will be within the city, and His servants will worship Him. They will see His face,