Because politics is not an industry, it's the unique dynamic of ruling and power. In this case: the politician is allowed to stay in the system because he enjoys enough political support, either by the public or through other key supporters.
This seems like a good opportunity to refer to CGP Greys "the rules for rulers": https://www.youtube.com/watch?v=rStL7niR7gs
Would be more honest for them to just come out and say, "as a collective we have some bias, and we have decided in certain areas that it makes the world a better place to amplify and reinforce that bias, including but not limited to: yyy, zzz." They could shape elections by their choosing, instead of elections being won by whoever uses Fb as a tool. They just need to come to terms with their place in the world exerting editorial voice, like the WSJ or NYT.
You are basically asking for the government to break you up if you do that. The only reason the giant tech companies are allowed to exist in such powerful positions is because they act as mostly objective third parties.
I disagree with the premise of your statement in so many ways. The tech companies arent special, and if facebook is actually a publishing company, not only does it have a ton of competitors, but there are other consolidating entities that are scary big, that the government allows to merge. Facebook should be broken up because, despite more options than ever for reading, people CHOOSE to consume it? It would make more sense to use tobacco company style arguments against Facebook, that it is targeting people with an addictive product that is harmful to health and society. If one bought into that argument, the remedy would be to make the class of their actions illegal, not split them in half. Two smaller companies with the same behavior wouldnt be better, and realistically there would be nothing stopping everyone from consolidating back to one of the two products.