This is actually a very astute question. A lot of people who are not writers might find it confusing, but many writers discover the same idea early on. Grades are kind of meaningless, as it’s the knowledge we care about. Degrees and pieces of paper are worthless to a writer, since the only thing that will matter for us is the quality of the writing. Readers don’t care if someone has graduated from college–they won’t even pay attention, most of the time.
There are a couple of things I’d like to say about this. First is, you’re right. You’ve figured out the game of academics–the truth is that the grades are, themselves, meaningless. A letter on a piece of paper doesn’t say how much you’ve actually learned, it only says you’re good at playing the game. Unfortunately, the game is really important in our society.
You ask why grades in anything other than English would be important–the truth is that grades act as a kind of pass key to get you into situations you want to get into. Good grades in a lower level class will get you into higher level classes, which is where you absolutely want to be as a writer. Many famous writers have noted that English is actually among the worst majors for writers, as the best thing to do is study something you’re passionate about, learn from the great thinkers, then apply that to your fiction.
A history major is going to make a better fantasy writer, often, than an English major. A physics major who trains themselves to write is often going to have a leg up in science fiction. (I say this as one who was an English major myself.) A writer should generally seek to know as much about as wide a variety of subjects as they can–give me a student who has studied psychology, geology, history, and anthropology, and I feel more confident in making a fantasy novelist out of them than anyone else.
Beyond that, a college degree is really handy for getting the kinds of jobs that will be useful for you to learn as a writer. Few of us start out as writers; for most, we do something else for ten years or so while practicing our writing. Having a good back up plan is really important for a writer, because you can’t know when you’ll end up breaking in.
So yes, academics is a game–and the grades are, to an extent, made up signifiers that you understand how to play this game. But learning to play the game will help you a ton in your career, will open doors for you, and will give you a good backup. I highly suggest going ahead and playing by society’s rules until you’ve mastered this game enough to start writing your own rules, via your fiction.
Hope that helps!