Adam lived for over 900 years, meaning he was alive during all generations previous to Noah. Do you find it strange that nowhere in scripture do we see any account of Adam preaching to his posterity? Isn't it strange that Enoch went on a mission to preach the gospel, despite the fact that Adam was still around and only middle-aged at the time?
Shem, son of Noah, lived for 600 years. He was alive both when Abraham was born and when Abraham died. He died well into Isaac’s life. Isn’t it strange that we have no record of his preaching to the people? Isn’t it strange that the only record we have of him at all is his blessing Abraham (if you accept that Shem is Melchizedek), and only after Abraham had completed so many of his experiences in life?
What were these people doing with their time? If they were preaching, why do we have no record of it? If they weren't, what were they up to?
Everyone who has light shares that light. Even if they don’t do it very effectively, they can’t help but radiate the light they have.
These holy men surely taught others. The fact that we don’t have their teachings is not a reflection on how they spent their time, but instead an instance of the principle of windows of truth.
When someone progresses in truth, they encounter higher blessings, but also separate themselves (to a degree) from those who possess lower grades or smaller quantities of truth. In order to progress to the level of law and glory (which are the same) that he possesses, the Father had to separate himself from those of telestial and terrestrial glory. Though he has constant and total connection to them through the spirit, he cannot occupy the same space with them. People also experience this, on a lesser scale, here on earth.
As a person acquires more truth, his capacity to preach it to those of lower glory/law without revealing a degree of truth above what they can bear is diminished. For every degree of glory, there is a degree of glory at which interaction is no longer possible without unavoidable damage. Were the Father to appear to a terrestrial person, the latter would immediately be destroyed. A person can obtain enough light that interaction with those of lower levels of glory will cause them to permanently turn away from the truth.
The more spiritually intelligent a person is, the more often their interactions with someone will be correcting in nature. Unfortunately, people don't want to learn; they want to feel good about themselves as they are; they want to be left alone. Juxtaposing these two opposites will help you understand the separation from normal society that comes as a person acquires more light and truth.
Why can't those who have more truth simply keep their mouths shut? Because a fundamental principle of acquiring truth is to always stand for it, at all times and in all places. It is so fundamental that it is the covenant we make at baptism (though few keep it).
The sons of Adam separated themselves from everyone else. They fled to a mountain where they could be separate from the masses. Enoch, who was raised in this separate environment, was called to go to preach to his cousins, those who had been born and raised in the inverse to his experience: in an environment absent from those who had followed God.
As you acquire more light, you are less able to teach that light to the masses, who generally reject higher order truth. But you will be able to help those who have acquired greater light. These are fewer, but those that can help them are fewer still.