YouTube is an integral part of sharing information, and the like/dislike ratio is a crucial yardstick of if the content is trustworthy. 95% of the content is not bullsh*t in my mind, and if it is, then why not have the dislike button anyway?
The like/dislike ratio only shows how many people (maybe) clicked a button. Nothing more. It has nothing to do with how trustworthy content is.
Infact I would argue that the moral thing to do would be to remove both the like and dislike count altogether. The like should just tell the system "Yes, I would like to see more content like this" and "No, don't show me content like this".
As far as why not have the dislike count. It isn't up to me or you. We don't own or control YouTube. But what we do control is our own actions and what we choose to use or don't use.