The only reason I can think of is to mitigate the downside risk of financial collapse. These are banks buying these bonds. Banks which might be worried that sho

The only reason I can think of is to mitigate the downside risk of financial collapse. These are banks buying these bonds. Banks which might be worried that short term financial pressures might tempt the governments might to reach for their cash positions. I would rather hold some negative-yield bonds instead of cash in that scenario.