Why Don't You Study?

So I'm a teacher at one of the top high schools in the country and all my students are very smart and involved. I have students who are absolutely excelling in the highest level classes in subjects they don't enjoy (mostly in the STEM area) but are just putting in minimal effort in subjects they are very interested in. Why is this? This is probably not all teachers experiences but it seems like many of my students would be satisfied with a C in their least favorite subjects but are content with getting A's in their favorite subjects. Why do you not put that effort into a subject you actually like? I hear so much complaining about putting in effort for classes you don't like but in many cases, my students will probably do so much better in a field they enjoy. It would be much more enjoyable to put in that effort for an enjoyable subject than sweating blood and tears for a subject you don't like and probably won't remember half of in 5 years. But I see some really bright kids just not putting in the effort. Why??