Chickenpox Parties have long been a thing[1], because getting young children catching that pox ought to be relatively harmless to them and prevents them first catching it when adults, at which point it can be far worse for reasons I couldn't tell you without looking it up.
I imagine that with this similar thing of the young (adults) generally getting away with few or no problems (that we know!) and most bad cases and deaths being in the upper-brackets of age, the logic (such as it is) is to try a similar trick.
Counter-arguments mostly arise from this being a novel outbreak, not one whose old-wife's-tale traditionalist non-medical approach to handling it has been put through the crucible of continuous checking against reality and increasing medical knowledge over decades and decades (perhaps centuries, possibly millenia) and has generally been found to be not a hidden dragon of a problem. If it is an outdated approach, with childhood vaccines now being an (at least) equivalently proven-beneficial thing to hold off the threat (for those who don't get innoculated against vaccines by the antivax misinformation), it is still a good second option.
With Covid, we just don't know, though. Young adults (and even young children) have been struck down by this - and not only those who are otherwise 'unnaturally' living with comorbid conditions known to interact badly. It's an age-band that the traditionalist would not suggest is safe for c.pox, as if that's any guide. The "catch it once, then never again" thing is unproven. The (more elderly) adults that would now come into contact with such party-goers have not themselves been party-innoculated when they were young[3] so cannot blithely supervise procedings or their aftermath, or even be around as the fizzling-out is still fizzling. Many more hidden variables remain so hidden.
And, moreover, the purpose is not an altruistic act by the elders-and-betters, working with principles passed onto them by their E&Bs, but is a self-guided possible-fantasy of freedom by the "I'm alright, Jack" generation that is deciding there's no real risk for itself (without following up the known repercussions, never mind the unknown ones) so let's PAAAAAARRRRRTÉ....
I'm not even sure there's any direct lineage to pox-parties, though it probably forms a sort of racial memory (an inkling, at least, rolled out as an explanation if challenged)used to 'justify' the naked hedonism at the core of the pursuit of such transient 'happiness'.
[1] Or were? I know of them mostly be reputation, TBH, as something that "has long been a thing" for the generation before me. Though I can't recall a specific party or two for me, immediately prior/post my own catching it[2], so was likely just an organic transmission process from what might these days have been called 'play dates'.
[2] If I bare my chest, and part the hairs, I can still see the most prominent 'scar' left over from the time, and remember the camomile lotion being rubbed into me by my mother. I was a mid-to-upper-single-digit age, for sure.
[3] Individuals who was never exposed to c.pox seem to be rare individuals, noted as needing to take care during "party season", in particular. If it's not actually a plot point in an Agatha Christie book, it's one in a AC-era book (by publication or just setting) along the same lines that I half remember, but results in an unpoxed person (for whatever reason they were) being vulnerable and knowing it.