Some people view their grade not as a measure of their understanding but as a measure of merely being able to manipulate a system to benefit them. While it is true that your grade should closely match what you know of the material (though, that isn't always so), some might be inclined to disagree, supposing that they know the material and thus scraping by will not affect anything but the immediate present.

Making the grade has very concrete effects, not the least of which allowing you to move on in your studies. Retaking classes cost more money, sometimes compounded by the university; sometimes it will delay your graduation, delay an easy summer with friends, etc. So being able to tell yourself that it doesn't matter, focusing on the grade for an immediate benefit rather than the material for the long-term benefit, is very easy to slip into.

Not everyone, of course, is lying to themselves when they barely scrape by but maintain that they know the material. Exams are meant to be hard. Some people know how to better manage disappointment. When getting my scores back on an exam, I know exactly how much I have tried to know the material, and will know that my score will not only reflect that but will not be changed. So there's no point in worrying about your grade after the fact, unless you just want to have anxiety problems.