That wouldn't be good, because that would deter people from rating it. Instead of clicking "dislike", they'd just leave. If I had to explain why I was rating something a certain way whenever I did, I'd never rate anything because it would be too much work. When people have a single-click option, they can express themselves that way without expending any time or effort to do it, so it is an attractive option for them. You don't want people watching your videos and then just leaving. Whenever people share, comment, like, or dislike your video, it tells YouTube that your video had an impact on the viewer, and YouTube doesn't consider any impact bad. So you may not like when people dislike your video, but YouTube likes your video better for it.