Because who needs actual effort when you can just say "Oh, great, thanks for nothing."
Example 1: "Oh, wow, I'm so sorry I'm late. Again." - The Human Version of a Broken Alarm Clock
Example 2: "I'm not arguing, I'm just explaining why I'm right." - The Rhetorical Equivalent of a Punching Bag
See More Examples » The Sarcasmictionary: A Dictionary of Sarcastic Phrases