They make assumptions and then foreground them so the reader can understand them. What's wrong with that? Have you discovered a way of drawing conclusions without assumptions? Or do you think you can "just observe" without being guided by theory? What would a non-"fabricated" model look like?
The problem is, that the assumption is not rooted in any empirical phenomena. They have neither historical data to fit to (which itself would be tenuous) nor do they have any sort of first order justification for it.
It's like doing a car crash safety simulation and stating that the airbag provides protective force "linearly proportional to the force of impact, with alpha > 1" ... without ever measuring it or doing any calculations on why this would be the case. Would you drive in a car that was tested this way?
I'm too damn lazy to read the paper, so you may be right. But normally, we practice a division of labour. Theoreticians build models in the expectation that empiricists will (a) test them and (b) estimate key parameters. If these guys have said "we have the answer, this is what you should do" then maybe you are right.
I'm not saying either that this phenomenon never happens. I just think it's rather rare, especially among sensible and skilled economists. See also the great Bob Sugden's paper "Credible Worlds", which is available here at the moment.
It's a paper on epidemiology, that utilizes econometric thinking. I understand the theoretician / empiricist divide very well. But I chose this paper to illustrate because it is very clearly aiming to impact immediate policy (it states this in the abstract). Furthermore it is not trying to create fundamental new theoretical models... but rather provide a justification for why lockdown is economically justified.
I hate to be the one to say this -- but most of modern economics is crap, given how reflexive it is. People know how the fed acts -- they know how policy makers act. The models don't take into account this reflexivity.
Someone like George Soros is a thousand times more savvy on how the economy works than post-docs publishing econometrics papers for $40k a year.
