If it is God's will for me to die in a car crash, why should I put on my seat belts? If it is God's will that I get healed, why should I go to the doctor?
You are assuming that God dictates every facet of people's lives. Some things God leaves to chance. "I returned and saw under the sun that - the race is not to the swift, nor the battle to the strong, nor bread to the wise, nor riches to men of understanding, nor favor to men of skill; but time and chance happen to them all" (Ecclesiastes 9:11).
It is God's will that you work for a living (II Thessalonians 3:10), but it is your responsibility to find a a job and do the work. It is God's will that you serve Him (Ecclesiastes 12:13-14), but whether you do is up to you.
It is true that Ahab thought to avoid God's prophecy of his death by dressing up as a common soldier. He was killed by a seemingly chance arrow that struck between the plates of his armor (I Kings 22:34). Ahab learned the hard way that God's decrees cannot be nullified by men. But to assume that God does this for every person on earth is a false generalization. For example, Esther has a choice of pleading for her people or not. What she decided would not change the ultimate outcome for the Jews, but it would impact her life and the lives of her family. "And Mordecai told them to answer Esther: "Do not think in your heart that you will escape in the king's palace any more than all the other Jews. For if you remain completely silent at this time, relief and deliverance will arise for the Jews from another place, but you and your father's house will perish. Yet who knows whether you have come to the kingdom for such a time as this?"" (Esther 4:13-14).
Satan once tried to get Jesus to sin: "Then the devil took Him up into the holy city, set Him on the pinnacle of the temple, and said to Him, "If You are the Son of God, throw Yourself down. For it is written: 'He shall give His angels charge over you,' and, 'In their hands they shall bear you up, Lest you dash your foot against a stone'" Jesus said to him, "It is written again, 'You shall not tempt the LORD your God.'"" (Matthew 4:5-6). One could argue that if it wasn't God's will for Jesus to die, God would prevent it. However, notice that Jesus said it is wrong to put God to the test.
If you take unnecessary risks, you are putting God to the test.