Because your mind operates within a more-or-less deterministic system.
If you are informed that someone is in need of help, you are ideally prompted to assist. The information that was given to you caused you to take action. How could non-deterministic behaviour improve such decision-making?
A decision is a choice. An action is not a decision. Non-deterministic behaviour allows for decisions, determinism does not. With determinism you only think you are making the decision, it is clearly an action that you can't stop caused by previous events.
In Determinism a rock slides down a hill because of all the previous events that caused it.
The same is with you. You help the person because of all the events up to that point force you to help the person. Like the rock you are unable to stop the slide.
This is Determinism, which is why you can believe it all you want it won't help you in life. What is determined to happen will happen no matter what. You can no more change my path than I can change yours.
This alone is why Free will is true. Without free will life does not matter, we are just cogs in a big machine clicking along unable to change direction or stop.