Thinking takes energy.

Like actual ATP molecules.

The world is complicated so we evolved these lumps of neurons on the ends of our spines to help navigate it. They are only as large as they have to be, and their primary purpose is to (re-)program the motor cortex and then go to sleep.

There is a strong evolutionary "pressure" to keep the brain in an "idle state".

If it seems like the evolutionary goals of the organism can be met with a simplified degenerate model then the organism will naturally adopt that model.

There is a possibility to notice that you're mostly-robotic and deliberately strive to become real but this is vanishing rare in practice.