Or put another way, humans can't be ready and wary, constantly and indefinitely. At some point, fatigue sets in. People move in and out of the organization. Periodic reviews of security practices don't always catch everything. Why something was implemented was forgotten by institutional memory. And then there's the cost for retraining people.
Also, those that are actively using mitigations that are going to be outdated at some point are probably far more likely to be aware of how close they are to being outdated by encountering more ambiguous cases, as seeing the state of the art progress right in front of them.
As for people sticking to outdated security practices? That's a problem of people and organizations being introspective and examining themselves, and is not linked to any one thing. We all have that problem to a lesser or greater degree in all aspects of what we do, so either you have systems in place to mitigate it or you don't.
To use a Go (the game, not the language) metaphor, skilled players always assess the whole board rather than automatically make a local move in response to a local threat. What's right for one organization is not going to be right for another. Asking the caller to turn sideways to protect against deepfakes should be considered within the organization's own framework, along with the various risks involved with deepfakes, and many other risks aside from deep fake video calls.
If there is no such framework, this is no different than yoloing lines of code in a production app by a team that does not have at least some grasp of the architectural principles and constraints at play. Or worse, not understanding the “job to be done” and building the wrong product and solving for the wrong problem.