However, in the UK at least, it can be extremely difficult to be conclusively far enough from all parked cars to ensure that someone flinging a door open won't hit you. Roads simply are not wide enough, and double parked streets of cars are ubiquitous, as are impatient cars who will harass cyclists trying to stay out of the door zones. On top of that, cycle lanes are often directly next to parked cars with little door zone allowance[1]. The bikes don't have to cycle in the lane, but if you don't, you may get punishment-passed by motor vehicles.
You also do not have to be going very fast to die in a dooring. Even leaving aside the effect of a metal and glass slab on a human face, car doors are actually extremely good at bouncing cyclists directly and rapidly into adjacent or oncoming traffic at almost any speed - even if the door hits the cyclist from the side by being opened as they pass.
It's also very unlikely that a bike injures someone exiting a car anyway, as opening the door and climbing out into the road are two very separate actions. If the bike doesn't see the door opening and someone getting as they approach, then it is indeed likely they were cycling unsafely. But that doesn't happen very often because that will also be very bad for the cyclist (as opposed to to car taking off a door and a person next to it, which doesn't injure the driver), and it would have been happening for the last 40 metres of the cyclist's approach.
It's also untrue that if you are driving down a narrow road with a single traffic lane and double parked cars (very common in UK urban areas) and someone flings a door open and you hit it that it's necessarily your fault. It's impossible to drive such that you could always stop in time, someone could even open the door while you're alongside when you're doing 1mph.