This is the fundamental question. Every YouTuber quickly figures out that the more extreme content you produce the more views you get. This is how something as silly (but harmless) as the Atheist Youtube community degenerates into idiots raging about feminists and denying 9/11. YouTube is just a market where the endless demand for extreme content can find a ready willing suppliers.
What it looks like is that YouTube is just a mass demonstration of the Fundamental Law of Group Polarization [1]: low cost communities must enforce high cost penalties. The easier it is to enter a community the more extreme the community must fight outsiders and punish internal deviancy -- otherwise the community will simply dissolve. This is the logical conclusion of Iannaccone [2].
YouTube communities, being based on videos that absolutely anybody can post and watch, is as low cost to enter as it gets. Given such bare minimum entry costs, YouTube viewers rationally want "proof" that everybody else (the video creator, the paying supporters and the supporters who just watch/subscribe/like) really are dedicated and it's worth sticking around in the community. This proof comes in the form of producing and consuming more quantity of video hours and more extreme content.
The genius of YouTube it that it implicitly understands the "race from zero" dynamic at work and every feature of the site is designed to exploit it. People get hung up on the algorithm but the algorithm is a very small part of it. Everything about YouTube: its anonymity, its system of subscribes and likes and the very prominent display of these numbers, the YouTube format which so often includes somebody staring directly into a camera trying to make direct eye contact with the viewer, the confessionary content, the RL social events -- invites users to prove that they are truly dedicated members of these low cost communities. (Google really missed the ball with paid donations, btw. The company is so hung up on advertising they didn't realize that YouTube viewers would leap at the chance to just give straight cash to their favorite creators.)
In conclusion (1) Google could turn off the recommendation algorithm tomorrow and it would at best slow down YouTube's extremism (2) there's enormous profit to be made given the fundamental dynamics of the site because (3) there's nothing unique about YouTube here, we see the same dynamics at work in politics, religion, sports, and plenty of other free sites like reddit and Facebook. Once you have people participating in these low cost communities the communities will naturally polarize in order to preserve themselves. This is not going to change any time soon, really, it's the new norm.
[2] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.475...