Because they have a deflationary view of the euro area economy. Now, investors are losing so much money just by holding German bonds in real terms (adjusted for inflation). Today, the Germany 10-Year bond yield hits 0% again.

Keep in mind that raising interest rates in the future will be painful for Eurozone bond investors. Secondly, extremely low interest rates are also bad for European banks, which in turn is bad for economic growth. It’s a feedback loop which could lead to a recession.