The universe giveth radioactivity, the universe taketh radioactivity away. Or at least, maybe the universe would be willing to accept it, right?
It's true that nuclear waste would be hard-pressed to do much damage to people if it were sailing through the solar system or, say, falling into the sun. The trouble is getting it there. Lift-offs fail occasionally -- catching fire on the launch pad, falling into the ocean, or exploding in the upper atmosphere. It's unlikely the failure fraction will ever go to zero. And until it does, shooting spent fuel rods into space remains a very dangerous proposition indeed.
Even if space launches were routine and safe enough to pitch our plutonium at the heavens, it may one day be worthwhile to recover this radioactive stuff. Plutonium, cesium and strontium are also limited resources, and if fission reactor technology advances enough, they could become fuel sources themselves. So maybe we want to keep nuclear waste reasonably handy.