It's a generic term, not a type of attack. The question is similar to asking why we haven't developed a defense against being blown up when explosions are old.

It's a generic term, not a type of attack. The question is similar to asking why we haven't developed a defense against being blown up when explosions are old. As long as there are services, and as long as those services have finite capacity, there will be DDoSes -- both accidental and intentional.