Not everyone is interested in every class they attend -- some are there to fulfill a requirement, others perhaps started out interested but decided mid-semester that the topic just wasn't for them.

Professors have lives, just like you and me¹, and some have a policy of allowing well-intentioned students who might otherwise not pass to receive a C+ for their attendance, homework, and class participation -- skipping entirely the final exam which would serve no purpose for the student and just take up the professor's time. To the professor, the Gentleman's C+ is an agreement -- you agree to do your best, and I'll pass you whether you master the material or not -- freeing up my time for working with students who are interested.

¹Well, just like you, at least.