This only works because YouTube stays focused on what you're doing without trying to build a bigger picture of who you are. It would be nice if it could somehow do both, but I don't have a clear picture of how that UI would function.

In other words it doesn't consider you as someone with a long history. You can change your profile from a conservative to a liberal in a few hours watching videos. Whether it's possible to have a balanced amount of crazy ( not the same as a centrist) is something I'm currently working on, it requires effort!

It's a classic race-to-the-bottom. If YouTube lets Facebook keep you addicted, you won't get to the long-term.

http://timewellspent.io/ is good for perspective

Personally, I find YouTube terrible at keeping me engaged. Even when I want to keep watching videos, their sidebar fails to show me anything I want to click on at least 1/3 of the times.

This is too fine a point. A user will "select" a default choice by doing nothing, and it's in YT's interests to provide default choices that reward the company. This has been researched for decades and is supported by, among other things: https://en.wikipedia.org/wiki/Status_quo_bias

But why? If I've watched hundreds of Zeni Geva videos over the years, but it's my fifth Tool video, why would I be more likely to click on a ad on a Korn video?

Of course it's possible they have, and realised they still make more money by focusing on the more recent videos.