Not necessary preppers as that is someone who's motivation is to mitigate some hypothetical future bad thing happening
I think most self-hosters are doing it out of a combination of technical exploration and mitigating real issues that exist today, e.g. cloud service outages or market exits causing something previously bought to be useful to become a temporary brick or permanent e-waste. Well, and cost in some cases, no one particularly enjoys having an extra bill for hosting.