Brian Jones from Microsoft explains:

Why does Office Open XML look like previous Office formats, wrapped in angle brackets?

It works best this way. Office Open XML needs to be as compatible as possible with older versions of Microsoft Office's file formats, and the best way to do that is to use a format with a similar design model. Since there are several million Office documents for every developer that's ever worked on Microsoft Office, the only practical way of ensuring compatibility with that huge corpus is by making cautious, incremental changes.