Often a task ends up requiring much more work than expected, or some more important task displaces sprint work, or you need to do something like prototyping or design work or customer consultation which can't be expressed well in terms of concrete deliverables. An environment where your boss is going to call you to account for only getting 2 points done last sprint when you were supposed to do 10 creates nasty incentives to avoid these things in favor of small easily-defined tasks.