I don't think it's wise to speculate, but the stabilizer is coupled to a very large trim wheel in the cockpit that both audibly clicks as it spins and is market

I don't think it's wise to speculate, but the stabilizer is coupled to a very large trim wheel in the cockpit that both audibly clicks as it spins and is market with white stripes (on a black wheel) to make the movement apparent. It's possible the pilots didn't realize what was happening, but it's not as if MCAS was silently causing a runaway.