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Because there are laws that specify when the brake light has to come on, and it isn’t when the car shows down (slightly).
To be clear, the laws say when it must illuminate. They do not (in the US) prevent illuminating it for other reasons in any way. The law says the light must illuminate/burn if you are actively pressing the brake pedal, but does not prevent it from also illuminating if a certain amount of regenerative braking is applied or a deceleration is detected. Theoretically an automaker could get away with making the brake lights simply always illuminate (and that loophole would be fixed in days, so no one does it).
Yep, EU as usual having reasonable and well-thought out laws, give the US about 5 more years and they’ll make it law here too.