When the brake fluid heats up due to the friction from the brake pads on the rotor being transferred to the fluid, the hot fluid will migrate towards the cold area, so there will be some circulation via convection currents. The fluid does not circulate like say the coolant but it does move around and migrate. Also, brake fluid is very hygroscopic and the system is not closed. The cap is open to the air. Brake fluid will gather moisture from the air that lowers the boiling point and can cause brake fade. Moisture in the fluid is also bad for the ABS and other components.