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).
That is indeed US-specific. I’m in the EU, and here it’s defined by when and how it’s switched. Specifically, it is required to be tied to the brake pedal (i.e. then intention to brake) and/or the hand brake being pulled. It is not allowed to illuminate otherwise. But the exact specifics probably also vary by country here. That’s why I emphasized that part.
EDIT: There are actually deceleration values in some laws, possibly tied to regulation of EVs and the regenerative braking. Since that isn’t necessarily tied to the brake pedal when silmulating engine braking, but can be adjusted in strength at will (it isn’t tied to the mechanics of the drag of an idling engine as it would with an ICE). A quick google told me that the lights are allowed to come on at 0.7 m/s² and are required to come on at 1.3 m/s². This obviously implies that they are NOT allowed to come on below 0.7 m/s². This still applies only to (pure) EVs, as far as I can tell (not hybrids, and not ICE powered cars).
