Joel Spolsky explains why Office formats are so complicated (and why that makes sense):

A lot of the complexities in these file formats reflect features that are old, complicated, unloved, and rarely used. They’re still in the file format for backwards compatibility, and because it doesn’t cost anything for Microsoft to leave the code around. But if you really want to do a thorough and complete job of parsing and writing these file formats, you have to redo all that work that some intern did at Microsoft 15 years ago. The bottom line is that there are thousands of developer years of work that went into the current versions of Word and Excel, and if you really want to clone those applications completely, you’re going to have to do thousands of years of work. A file format is just a concise summary of all the features an application supports.