I don't need to rationalize what is already rational. My degree isn't even related to my job, so why would working harder at it have paid off?

In short, soft skills. It depends on your degree I guess. In an MBA or education program, the distance between try hard and slacker is narrow. In more abstract less career oriented programs, the difference on critical thinking and comm skills is huge. If you can't imagine how deeply studying great works and thinkers improves your mind, that tells me maybe you missed something big.

I'm a software engineer. I have no CS degree. The people who have one aren't any better.