Why are people who have never had the Bible still without excuse before God?
Because God has given all people a witness of Himself: He made the world, gives life, breath, and all things, and set the times and boundaries of nations so they should seek Him and find Him. Even those without written Scripture should have known the Creator God, including through the original message in the stars.
Les explains that Paul was speaking to pagan Greeks who had no exposure to Judaism and no written Scripture, yet they should have had a knowledge of the Creator God behind all things. God gives to all, not just believers, life, breath, and all things; He made all nations from one blood to dwell on the earth, and He determined their appointed times and boundaries so they should seek the Lord, feel after Him, and find Him, though He is not far from any of us. Les also notes that Scripture indicates the Word of God was written in the stars once, that Adam and the pre-flood people could comprehend it, and that although Satan adulterated it, the pre-flood patriarchs had a good understanding of the Word of God by studying the stars.
God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;
See the rest