Its probably because extremists don't feel comfortable expressing their views in their everyday life, so they do it online instead, where they have some anonymity or at least don't face the same backlash as they would if they were to voice their opinions at work or at a bar or something.
Also, controversial takes get more engagement, which means more ad revenue.
In the past if you were a "weird person" you would not find too many people that resonated with you and eventually many would just "normalize".
Now doesn't matter what kind of shitty take you have, there are always some group of people who thinks the same and will echoing each other