Remember that Twitter gets something like 500 million tweets per day. If it took someone working minimum wage 15 seconds to decide whether or not a tweet violates their ToS, Twitter would spend 30 million dollars a day on this, and the results probably wouldn't even be that good. So they don't do that, instead focusing resources where they will have the most impact.
I am also pretty sure they are not trying to censor conservative viewpoints. If Joe Biden starts telling people to go shoot looters or that Mitch McConnell murdered one of his aides, I am sure they will add a little note to those tweets.