Thank you for the interpretative question, “Why is the Bible all about telling people how they’re supposed to be?”
If I may make an analogy, the Bible is like the Driver’s Manual for those who wish to drive a car. We would not imagine giving someone a key to car who hasn’t read and pass the Driver’s Test. Each driver must know the rules and signs so that s/he would be a safe and courteous driver. Thus, in the same way for a believer who becomes a child of God, s/he also needs to know the beliefs about God and about how to live the Christian life.
Every Christian wants to know how to live a godly life that is pleasing to God. The Gentiles believers needed to know how to conduct themselves in contrast to the non-believers. Some believers even believed that it was permissible to behave like the Gentiles. The writers of the Scripture had to teach them.
The Apostle Paul says this in II Timothy 3, “You, however, know all about my teaching, my way of life, my purpose, faith, patience, love, endurance, 11 persecutions, sufferings—what kinds of things happened to me in Antioch, Iconium and Lystra, the persecutions I endured. Yet the Lord rescued me from all of them. 12 In fact, everyone who wants to live a godly life in Christ Jesus will be persecuted, 13 while evildoers and impostors will go from bad to worse, deceiving and being deceived. 14 But as for you, continue in what you have learned and have become convinced of, because you know those from whom you learned it, 15 and how from infancy you have known the Holy Scriptures, which are able to make you wise for salvation through faith in Christ Jesus. 16 All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, 17 so that the servant of God[a] may be thoroughly equipped for every good work” (NIV). See also Romans 12:1–2.
Summary: The Bible teaches a believer how to live a life that is pleasing to God.