I tend to think of the drivers, not as reacting to threats, but navigating an evolving mental model of their surroundings -- with a primary focus on opportunities rather than threats. The mechanism that updates that model doesn't have "training data" with cyclists in the picture.
Probably because of poor training and infrequent interactions with cyclists/pedestrians. If something isn't a part of your regular life, and you haven't been told you need to watch out for it, then why would you?
In some ways cyclists are slow enough compared to how fast a car can accelerate. Even a moped or motorcycle can use speed assert its presence and maintain safe stopping distance between other cars.
Especially in a urban area where everyone is stuck trying to get to home/work/etc, most cars appear to be looking for opportunities. Opportunities to get ahead, to squeeze through a left turn lane before oncoming traffic, to find an empty bike lane to drop off an uber eats order. With that opportunity, get ahead” mindset, cyclists do not register. Until, of course, something happens.
