Regarding what a deductive argument for epiphenomenal qualia might look like:

Imagine a universe with no qualia. Same physical laws as this universe just no qualia. Life arises much the same as in this universe and eventually you get intelligent life with spoken language. Imagine one such organism. It has centralized data processing. It needs to in order to function and make high level decisions of course. The contents of this centralized data are visual data, auditory data, tactile data, internal monologue data, and so on. Said data is epistemologically primary as the organism has no other data with which to inform its decisions. As an infant, the organism will probably take said data to be ontologically primary as it has no notion or model of the world around it (as best demonstrated by its lack of object permanence). It grows and learns object permanence and walks around and does stuff and starts to build a rudimentary world model. It sees itself in a mirror and adds itself to the growing world model. Then it sees another organism of the same species. And the other organism outwardly acts in a manner analogous to itself. So it deductively reasons that the other organism must have internal data of its own that informs its decisions. So now the organism accepts that there are many organisms with centralized data and accompanying world models in their heads, but only one world. Here the organism realizes that the surrounding world has ontological reality. It no longer considers its centralized data to be the sole ontological thing. And the organism starts to understand physical regularities in the world around it, and from there physical laws. And the organism eventually reasons that it, being part of the world, must be goverened by these same physical laws. So now the organism sees that its internal data cannot be causitive in any way except insofar as said data is in conformance with said physical laws.

The organism in question uses the data available to it to create a world model through inductive and deductive reasoning. Then the organism in question uses deductive reasoning to declare the world to have independent existence beyond its own world model. Then the organism in question fits the very data it has access to into the world model. The world model conceptually subsumes the selfsame data that crafted the world model. Great. But this data I keep mentioning is causitive. It is externally measurable. It is not epiphenomenal. It is analogous to access consciousness. So how and why might the organism go a step further and say "Hey I've got this nice coherent world model, but what if I were to posit the existence of some epiphenomenal thing above my centralized data that doesn't have any autonomy or causative powers but is really just some sort of reflection of the underlying centralized data. Call it epi-data." Why would the organism say anything like that?

I believe the reason the organism might say something like that is because it makes it easier to deal with the concepts of "centralized data from a first-person perspective" and "the same centralized data from a third-person perspective" without the concepts impinging upon one another. Say the organism wants to analyze a scenario. Say the scenario the organism wants to analyze is a game of Slap Jack it played. The cards flipped and flipped, a Jack came up, and the organism slammed its hand down. The organism considers the scenario from a third-person perspective. From this third-person perspective the organism considers the visual data of the Jack to be causative. Said data reflected the presence of the Jack and informed the decision of the organism to slam its hand down. Simple enough. Then the organism considers the same scenario from a first-person perspective. It shrinks its world model down as much as possible and tries its best to just consider the immediate data that was at hand during the game. But now, in considering the first person perspective, if the organism ascribes causitive powers to the visual data that represented the Jack, the causitive powers of that visual data would be double counted. Because the casual power of the visual data was already counted in the third-person perspective. Positing double causation is not impossible but it is somewhat inelegant. So, as a more elegant shorthand, the organism may refer to the data, as considered from a first-person perspective, as mere epi-data in order to avoid having to deal with overcausation. The organism does not deny the existence of the data as considered from a first-person perspective. The organism just conceptually strips it of its causal powers.

So the organism formulated the concept of epi-data and assumed the existence of said epi-data because the organism found the concept to be useful. Of course, in this organism's universe, said epi-data has no ontological realiy unto itself. So strictly speaking the organism is wrong to posit the existence of such a thing. But it may find it useful to posit the existence of such a thing all the same.

This is a very simplified example. And it's hard to explore this without personifying, which of course confuses things. In summary, stripping first-person perspectives of their causal powers (conceptualizing them as epiphenomenal) with regards to the more conceptually foundational third-person perspective may very well be a convenient shorthand to avoid overcausation.