Because at full load your system will draw 33% of the output of that PSU. At idle/web browsing it will draw 8%. Efficiency?
That's the PSU that you buy to run three 980s, or three 780TIs, or two AMD 295s. A single sub 200W GPU? Why? I agree that you have no bottlenecks with that CPU, but the PSU isn't overkill, it's ridiculous overkill.