It still maintains their market position, which has value. For example, you might not visit other sites because they don’t have the content you want (and the content stays on YT because they have the viewers), or you might even share YT links to other people.
I think the main problem is that Chromium still contributes towards the browser engine monoculture, as it is bug-for-bug compatible with Chrome. Therefore if you switch to Chromium, it’s still enough for the web sites to test for Chrome compatibility, which they will, because it has the largest market share. Users of competing browsers suffer, further driving the lure of Chrome (or Chromium).
On the other hand, if people switched to some other engine, one that does not share the same core engine or even the same history, this will no longer hold: web sites would need to be developed against the spec, or at least against all the browsers they might realistically expect their customers to use.