They're also designed to be easily supported by other, non-Microsoft programs. In fact, they're made up of existing, open file formats. If you don't believe me, try this: Rename a .docx file, changing the extension to .zip. Then double-click it.
Yes, a .docx file is really a .zip archive (I told you it was compressed). Most of what you find inside are .xml files--another open standard. That experiment will also work on .xlsx and .pptx files.
After you're done experimenting, don't forget to change the extension back to .docx.