In real life, if you've got extreme racist views, you can go hang out with other racists. But to find these like-minded racists, you'll have to churn through lots of potential racists - who may often turn out to actually be anti-racist - to find your group to hang out with. In the process of doing so, a lot of people are going to question why it is that so many people dislike their racist views. And the ones that stick through it are going to know that they hold pretty extreme views relative to the rest of society.
In contrast, the existing social media platforms have a way of making it feel like you're right, and a lot of people are supporting you (doesn't matter whether it's 1% of society, or 99%). Inside your filter bubble, you're not an extremist, and you don't have to encounter very many anti-racists on your search for like-minded racists. When you do, you've got your fellow racist keyboard warriors backing you, and some people even enjoy getting into these arguments.
I think the system you're proposing works well for hobbies and interests like tech or sports. If people who play football aren't aware of what's going on in the world of soccer, it doesn't really matter a great deal. But society - at least a democratic society that depends on informed voters - is worse off if a large number of people get together and encourage each other to think that Hillary Clinton is running a pedophile ring in the basement of a pizza store in Washington.
tl;dr - because it encourages political polarization, which is bad for democracy
In contrast, the existing social media platforms have a way of making it feel like you're right, and a lot of people are supporting you (doesn't matter whether it's 1% of society, or 99%). Inside your filter bubble, you're not an extremist, and you don't have to encounter very many anti-racists on your search for like-minded racists. When you do, you've got your fellow racist keyboard warriors backing you, and some people even enjoy getting into these arguments.
I think the system you're proposing works well for hobbies and interests like tech or sports. If people who play football aren't aware of what's going on in the world of soccer, it doesn't really matter a great deal. But society - at least a democratic society that depends on informed voters - is worse off if a large number of people get together and encourage each other to think that Hillary Clinton is running a pedophile ring in the basement of a pizza store in Washington.
tl;dr - because it encourages political polarization, which is bad for democracy