There won't be one single reason. For some it is a dark sense of humour perhaps twisted a little too far off track, that perhaps they should keep in their own head or at least just between very close friends. For some it is simply money without caring that it might upset people: get enough engagement and ad impressions and it is worthwhile if you can ignore the moral aspect. Money might not be the objective at all, there are people who just want the attention, or the appearance of attention, and fake internet points (youtube views and such) sate their need at least temporarily. For some it is simply deliberate griefing, for all the reasons that is a thing generally. Or some mix of the above. None of it healthy IMO, but explainable.

In a few cases it is a dark in-joke between a small set of people that just happened to have used a public host for distribution, that unexpectedly went more viral.