The models are insufficiently multidisciplinary and some of the needed expertise is almost entirely outside of academia and scientific circles whence expertise is drawn to create the models. This is really evident when you look at the epidemiological models that were actually used.

What you find in practice is that the quality of the myriad sub-models that make up the models is extremely uneven, and reflect the expertise of the people building the model, both good and bad. Some key elements of the models are expertly designed, but other critical elements are so naively designed for lack of domain expertise that it completely destroys any signal one might hope to find from the expertly designed parts. This is unfortunately endemic, I've seen this in every model I've been asked to look at.

As for why multidisciplinary expertise is not used to build these models, in my experience there is a lot of gatekeeping against non-academics, even though critical state-of-the-art modeling research and know-how on things like population-scale behavioral dynamics is done almost completely outside academia.