Because they're huge and also have lights that don't strobe and, although their lights are bright, none of them are blinding. The combined effect is to make the vehicle unmissable, where as a blinding flashing light on a cyclist is infuriating and can maker it /harder/ to see where they are or where they're going.

Also, a commuter on a bike is just another road user where as a public service vehicle should get priority.

Don't get me wrong, I'm all for making ourselves as visible as possible, but flashing ultra bright lights are not helpful.

Keep it on full and point it at the road ten feet ahead - much better for everyone.