Well partly because certain things aren't counted in it. Take for instance Blogs. Some of the latest blogs have anything up to seven likes. Older ones have more.
One of the things we need to consider when we think about the popular content is what actual content it reffers to.
I at the very least always forget to like stuff, but I did back on the Blog as well so that's not really saying much. But honestly I kind of like it more, as people are more likely to comment advice or praise, which is nicer than a like in my opinion. Makes stuff harder to rank sure, but I still like it.