Why is it not mentioned in the Bible that Isaac allowed his father to sacrifice him to God?

The Bible does. It just requires thinking about the account.

This is what the Bible tells us

“Then he said: “Take, please, your son, your only son whom you so love, Isaac, and travel to the land of Mo·riʹah and offer him up there as a burnt offering on one of the

mountains that I will designate to you.”

“So Abraham got up early in the morning and saddled his donkey and took two of his servants along with him and his son Isaac. He split the wood for the burnt offering, and then he rose and traveled to the place that the true God indicated to him. 4 On the third day, Abraham looked up and saw the place from a distance. 5 Abraham now said to his servants: “You stay here with the donkey, but the boy and I will go over there and worship and return to you.”

6 So Abraham took the wood for the burnt offering and put it on his son Isaac. Then he took in his hands the fire and the knife, and the two of them walked on together. 7 Then Isaac said to his father Abraham: “My father!” He replied: “Yes, my son!” So he continued: “Here are the fire and the wood, but where is the sheep for the burnt offering?” 8 To this Abraham said: “God himself will provide the sheep for the burnt offering, my son.” And both of them walked on together.

9 Finally they reached the place that the true God had indicated to him, and Abraham built an altar there and arranged the wood on it. He bound his son Isaac hand and foot and put him on the altar on top of the wood. 10 Then Abraham reached out his hand and took the knife to kill his son. 11 But Jehovah’s angel called to him from the heavens and said: “Abraham, Abraham!” to which he answered: “Here I am!” 12 Then he said: “Do not harm the boy, and do not do anything at all to him, for now I do know that you are God-fearing because you have not withheld your son, your only one, from me.” - Genesis 22:2–12

From reading the account we know:

1. Isaac was not a young child. For he was transporting the wood for the fire. That is to heavy a load for a child.
1. We also need to remember this: “ Abraham was 100 years old when his son Isaac was born to him.” - Genesis 21:5
2. Abraham bound his son hand a foot. The young man who was strong enough to carry the wood could easily escaped his father and avoided being bound if he did not willingly allow his aged father to sacrifice him.