Because nobody is running these social media platforms with proper framework towards the goal of achieving such ideals.
What you are describing is a second-stage natural conditions of humans. What it needs is a push toward the third-stage. I recommend reading late Scott Peck's The Different Drum to understand this missing psychological framework.
Humans are naturally conditioned to be stuck in this second stage where nobody is nice. At first, this is better than being in the first stage, where everyone is nice. The problem however is, that the second stage needs a push towards the third stage (which is where the sanity prevails and people understand each other better).
The social media platforms are geared towards financial motives. If they were to gear towards human motives, they will implement some kind of framework (what Scott Peck did in his days).