So I have been using this website: https://apstudent.collegeboard.org/exploreap/ap-and-your-future to choose what AP classes I should take next year. However as I look into the computer science majors/careers, I see AP Music Theory on there numerous times.
Though I will most likely not take it, why is it on the list? I am just wondering.
Trying various college majors shows some inconsistency. For example, some majors list fewer than may be useful (e.g. computer engineering) while others list more than may be useful (e.g. computer science listing all four physics APs).
Probably because it requires logical thinking in an abstract Language.
I once did the Google search “AP class” “regret”, and AP Music Theory came up repeatedly.
Maybe College Board is trying to get more students to take more AP exams. Under Computer Science for career area, it lists 33 APs, almost every single one available. This College Board tool doesn’t seem to be very useful.
At the risk of not precisely answering your question:
Quite a few software engineers are very good at music. I do see a very significant overlap between the skills needed to be good at music versus the skills needed to be a software engineer. I know several software engineers who play in public and are very good.
However, AP music theory is not an easy class at all. I know someone who took it and did very well (A+ grade, 5 on the AP exam). However, she was both a very strong student and an exceptional musician. With this, the class was still very difficult and a lot of work.
Music and software both have a lot of left brain activity.
Way back when I was just starting out in the world of information systems, numerous companies would hire music majors as programmers. Music composition and software coding requires a lot of the same type of brain processing.
Personally, I think just because it’s rigorous. I’d have to stretch to find the relevance to stem. I think it’s just CB offering a range of courses a student “might” consider. Studio Art is listed under CS, too. I don’t know too many kids who think of art as a mandatory.
Right, take CB with a grain of salt. They list APMT for a classics major, as well.
Well, that was your first mistake. 
Why would you take course selection advice from an organization that seems to exist to nickel and dime you and your parents? Your GC (or even us) would be a better help.
But to answer your question (sort of), I know many CS majors who are also musicians. I’m a STEM major (but not CS) and a musician. Some believe that the left-brain/right-brain dichotomy is too simplistic. Who knows/cares? Bottom line, if you want to take APMT, take it. But if music is not in your wheel house, run as fast as you can away from this class; there are easier ways to fulfill an arts credit.
And not to be a broken record, not every course one takes needs to be AP.