I guess I should rephrase that: Why is the media so very focused on how much a movie makes during its opening weekend?

In general the curve of how a movie does under normal circumstances would start off a little slow and then pick up as more people see it and like it.

The only reason why opening number would be so important would be because they know its a shabby product and the opening weekend is the most traffic it will see. Once word of mouth spreads nobody wants to go back.