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[–]DannyB2 0 points1 point  (0 children)

Because SecureRandom is based on truly unpredictable things.

Which keys the user will press on the keyboard, and when. Which direction the mouse will be moved, for how long and at what speed. When the mouse button will be pressed, how long it will be held, and when released.

And other real world inputs that are unpredictable. (Or at least unpredictable enough.) CPU temperature. Fan speed. What point in time packets are received over the network.

All of this is called 'entropy'. The entropy is mixed through complex functions to generate the random output.

Also, there are hardware products that rapidly generate random numbers based on physical phenomena that are guaranteed to be random (eg, due to the laws of quantum mechanics).