Maybe because it's supposed to be a sort of summary of all the important/meaningful things that happened in the year, they are trying to focus our attention on the fact that all these things happened just in the span of the year (2007, this time). They aren't trying to give us a history lesson, as to what exact date each event happened. It also looks nicer artistically to make a sort of collage, without it looking like a history lesson. I don't know, I'm just brain storming :) Happy New Year to you too.