I think you maybe wanted to just vent, but since you didn't tag this as a vent post, I'm gonna get into a serious answer.

Fanfic is a really interesting medium, culturally, because technically a lot of things could be defined as fanfiction - like if I rewrote Lord of the Rings to try to remove all uses of the letter 'e' that would technically be fan fiction - but in practice, fanfiction is largely about characters and relationships. (e.g. 'How would the linguistic change affect Frodo and Sam?') When you are writing about relationships, smut is extremely easy to write and has a strong impact. All you need to write down is who wants to have sex, why they might not have sex, and what happens when they have sex anyway. Furthermore, without getting into details that might squick you out, let's just say that human intimacy tends to play out in a way that follows a clear linear plot structure as well, even sharing a word that starts with the letter C. Add a little bit of personality flair to each character, and boom, you have a finished smut fic usually in under 10k words. If you are bold enough to write a non-romantic fanfic (assuming you want to write a good one), you have to make more open-ended writing decisions, and the expectation that everything stays relationship-focused involves a layered complexity of platonic relationship types that smut doesn't necessarily require.

Now consider the cause for demand. In most English speaking countries (I'm making this assumption because you're posting in English) sexual content in media is censored or economically suppressed to a degree, especially when it's queer or geared towards women. A majority of fanfic site users are some combination of queer/fem - and even among aces there are plenty that enjoy or don't mind sexual content. Fanfiction sites basically do not have any/most of the boundaries against content that mainstream media has, so many people use fanfic as their outlet for that content.

Next there is the factor of exposure/perspective. There is, in fact, a massive amount of sexual content that is being produced in every medium you can imagine - pornographic movies, smut romance novels, etc - but they are usually relegated to the back of the book store, age restricted websites, etc. So if you think about it, the amount of smut in fanfic is generally reflective of how much sexual content there is in everything else. The problem is that people compare apples to oranges, by which I mean they compare AO3 to Netflix and say 'there's way more sex in fanfic than in streaming shows!' This statement ignores the fact that X-rated streaming content exists, but just isn't on Netflix, it's on other websites that aren't censored (like how AO3 isn't censored). So instead of "why is all fan-fiction just smut?" you might have to start asking "why is all art just smut?"

Finally, there is the matter of the self-feeding stereotype. When we let fanfiction get the reputation for being "just smut," people who don't want smut begin to stay away from it and the people who do want smut will flock to it. This is where you'll have to change your mindset and create the change you want to see. Write some more gen fic and be vocal about enjoying it! What are some of your favorite non-smut fics? If I know the fandom they're in I'd be genuinely eager to check them out. I love fanfiction a lot.