Like the MS documents it is not meant to be parsed into memory but directly dumped from the file and ready to go, they used this in a time when computers were too slow otherwise to do meaningful things to documents.
Due to their success in these earlier years they never had time or lust to change this into a proper parsed format like most file formats nowadays.
A case where security by obscurity ensures the future of the product, for now.
I guess this is similar to Facebook / g+ / etc refusing to have easily imported / exported data - if you can export your data to a useful format, you can move it to a rival product.
Facebook doesn't have, and won't ever add, an 'export my photo albums, messages and friends list as a series of easily parsable xml files' button, because that would be building redundancy into the product. One of the ways that it locks you in is to to secure all this data, all this work that you've done. The fact that your friends all use it is obviously another significant factor.
Similarly, I CBA to switch from Spotify, because I've got all of my playlists saved in it. If I moved to google play or one of the other up and coming music services, I'd have to rebuild these manually. There are some scraping tools to get this data out but they tend to be painful - "Just right click on each song and click 'copy HTML link'" etc ...
Actually Facebook has, and has had for the past few years, a "Download your data" option within the "Account Settings" page. That system collects your posts, messages, friend list, events, photos, and albums into one zip file. The structure isn't terrible (HTML), but it changed fairly often.
I still have a little "battle fatigue" from writing an RTF converter about 15 years ago.
It went from "Oh, this standard is pretty simple. I'm actually impressed that Microsoft made a reasaonble standard, that's pretty cool" to "OMFG what have you done to my youthful innocence?" -- importing tables was one of those exercises where you put your pride on the shelf and just write the code in front of you.
"Okay, Microsoft. You want me to launch COM now, don't you? Fine. I'm down here in the muck, wallowing hip-deep in the filth, so what's one more flush of someone's toilet of an abstraction to me? I am invincible. Some day I will rise from this hole, ship the steaming mass I had to wrap around your already stinking soul, and know RTF no more."
