It's perhaps more useful to see these as (frameworks for) normative theories, describing which decisions are better than their alternatives in certain situations, analogously to how laws of physics say which events are going to happen given certain conditions. It's impossible in practice to calculate the actions of a person based on physical laws, even though said actions follow from physical laws, because we lack both the data and the computational capabilities necessary to perform the computation. Similarly, it's impossible in practice to find recommendations for actions of a person based on fundamental decision theory, because we lack both the problem statement (detailed descriptions of the person, the environment, and the goals) and computational capabilities (even if these theories were sufficiently developed to be usable). In both cases, the problem is not that these theories are "impossible to implement in humans"; and certain approximations of their conclusions can be found.