If they flicker it's because the designers/engineers did something wrong. It's an entirely DC system, so the only way they would flicker it's if they're using PWM control for the intensity of the lights. There are better ways to dim LEDs, which don't introduce flicker.
Yep, because they're cheap. I don't notice it anymore, which only tells me that things have improved, I used to see it a lot with brake lights. Not so much with headlights.
I dunno why they're horrible, I just know that's an effect of PWM control that's been set up pretty poorly.
I'm not an engineer, nor do I work on cars, either designing them or fixing them. I'm just some guy.