The writer of Hebrews said something that touches on this topic:
11We have much to say about this, but it is hard to make it clear to you because you no longer try to understand. 12In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God’s word all over again. You need milk, not solid food! 13Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. 14But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil. (Hebrews 5:11-14)
God does not reveal all things clearly and quickly to a person, no matter how faithful. There are basic matters related to salvation, then there are matters of wisdom that we only master as we progress in faith. For example, take the issue of whether women are called by God to serve as pastors and elders. I held one view on this question for about thirty-five years. I read articles on the subject, sat through church discussions, debated with people and prayed over the matter. Then three years ago, I prayed again about this and the Holy Spirit led me to passages that I had overlooked or not understood properly. A week later, I did a 180 and reversed my position. I am in my sixties. I took classes at seminary, have read widely on theology, and have written books on Christian topics. Why did it take so long for me to hear the truth? It is because of spiritual deafness. That is the general answer.
However, concerning the topic of Young Earth Creationism, there is a more specific reason why people can go long years holding to one view or another, when the opposite is correct. That reason is time. God has reserved for his apostles and prophets, and in many instances Himself alone, the knowledge of time.
When will Christ return? Jesus told us this:
"But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father." (Matthew 24:36)
Solomon's greatest treatise on wisdom, Ecclesiastes, is relentlessly focused on time. It is structured to show that our path through life passes through many milestones. The twenty-eight times of Ecclesiastes 3 carry us from birth to peace, achieved if we reach full maturity. For example, the times to weep, laugh, mourn and dance are the teen years, when kids need to learn verbal and physical self control. Then the times to gather, scatter, embrace and refrain are about forming and breaking friendships and finally getting married, while the times to search, give up, keep and throw away are about making good carer decisions and managing your resources well. Chapter 10 speaks of the productive middle years, when a successful person builds their house (either a literal building or a family legacy). It also talks about wise, diversified investments when it says to cast your bread upon the waters. The last few chapters speak of the years of decline and death.
Solomon is teaching us that wisdom is all about knowing the right and wrong time to perform a given task or pursue a goal. His book also tells us that only God is able to plan out history from beginning to end. Only by growing in wisdom, which requires suffering, perseverance, and lots of prayer and study, can we attain a small measure of wisdom about time.
Prophecy is all about time. To properly interpret many prophecies, you need to know the precise years when certain past events occurred so that you can count out the years until some future event shall occur. Some prophecies may count time starting from the date of creation. Thus knowledge of the timescales of the true history of the earth will be necessary to solve these mysteries. God measures out understanding of such matters to very few.
Revelation speaks of a scroll with seven seals. As each seal is opened, plagues and disasters are set to occur. Many have speculated about what the plagues are and when they will occur. There is something more important than that, though. What is on the scroll? It seems that as each seal is cut, new knowledge will be revealed to the church. It is likely that part of that information will be knowledge about what will happen during the time remaining until Christ returns. If this is so, then the ultimate reason that so many people are confused about the age of the earth and related matters is that the Lord has not unlocked that knowledge yet.