Musk is a libertarian utopian-capitalist. He believes that humanity's lot and chances of long-term survival can only be improved by unrestrained, accelerated economic (and scientific and technological) growth. He's an Iain M Banks fan, and I think his ideal is probably reaching a sort of hyper-advanced, post-scarcity civilisation managed by benevolent AIs.

Therefore, from his perspective, the current lockdown is putting a huge brake on the progress necessary to help humanity and eventually reach utopia. Of course, more concretely, it also endangers his fortune and life's work, but people don't tend to like thinking of themselves as selfish to the point of acquiescing to mass death, so I imagine he rationalises his opposition to himself in this greater-good sense.