That's the problem. Your brake lamp might have 15 LED elements, and if only one of them is burned out, it's brightness is not dimmed appreciably (1/15th, or 6% dimmer, to be exact...).And at least in my state, all of them have to be working in order to pass the mandatory annual safety inspection.
The state inspection people should recognize this and set a percentage allowed to be burned out (say, under 10%).
The good news is that LEDs are inherently more reliable than bulbs. The bad news is that automakers are so cheap that they don't specify the top-quality LEDs.