The amount of money spent to send New Horizons to Pluto is a drop in the bucket compared to other expenses of government. Really, you could plop anything in that statement "Why spend millions on war while...", "Why spend millions on campaigns while...", or "Why spend millions on corporate tax breaks while...".
The reality is that there is more going on than just world hunger. Maybe if we put out minds to it, as a united front, we could solve the problem. But one country can only multi-task so much. And, as far as frivolous spending goes, space exploration is the least of our concerns.