A straightforward answer to the question "why do some people sometimes like to connect over niche things, specifically," is that people sometimes feel lonely, they crave connection with other people, and one of the best ways to experience connection is to share a common interest. When the interest is niche, the feeling of connection is amplified.
Let me explain. Originally I enjoy a thing (good), but it's niche, so I enjoy it alone (bad). When I share it, I get connection (good), but I also get to enjoy it with someone, which means I no longer enjoy it alone (good). Basically, enjoying something niche specifically comes with the curse of loneliness, but the curse can be annihilated if I'm lucky enough to find someone else with the curse. On the other hand, enjoying something mainstream does not come with the curse of loneliness, but it also doesn't have the same potential energy as a niche thing.
I think some of your bitterness can be explained as follows. When you see people connect over niche things it can amplify your own loneliness, because they are sharing connection that you cannot participate in because you don't share the interest. The good news is that there are so many other ways of connecting besides sharing niche interests, and so it's ok that you don't share them. Also it feels good to share mainstream interests too! So connect with drake fans over your enjoyment of his music! And if you want to connect with someone who doesn't like drake, looks for other ways to connect! And if someone is toxic about your interests, avoid them!