Twitter doesn't make enough revenue to hire enough people to do that intelligently.
You can't operate a chemical factory without paying for adequate measures to prevent spewing toxic waste into common resources like air and water, so why should you be allowed to run a facility that spews toxic hate into societies without establishing adequate protection?!
If there is one thing that made the internet so toxic, it was services labeled as "free" that in reality were nothing more than avenues for advertisers to shove ads down our throats, with the cost being our societies breaking apart as a result of all the lone village nutcases suddenly unifying themselves and no one paying for the virtual equivalent of the police to make sure the nutcases don't create chaos.
I think they're trying. ;-)
To be clear, I'm not defending Twitter. If I was in charge, I would actually shut down Twitter for the benefit of the world. My point is just the basic financial fact that Twitter doesn't have enough money to do this, and thus they won't, indeed can't.
To cite a particularly egregious example, I once saw a person banned for "giving medical advice" on /r/adhd, while not one, but multiple other moderators had also given medical advice in the exact same thread. A thread that was explicitly asking for medical advice.
I don't believe that anyone could moderate at Twitter's scale. More than 500 million tweets are published per day. How do you moderate that? It's like bringing an umbrella to a tsunami.
Not to mention, Reddit moderation is crap. I've tried to use Reddit several times but gave up each time, because my innocent comments — mainly writing technical help — were getting moderated. I wasn't writing anything political; I couldn't even post on r/apple! They still use a lot of automation on Reddit in the bigger subreddits.