The sender may be revealing more information than they thought. Many versions of Word have a habit of revealing details about your computer that you usually do not wish to share. Word files can transmit viruses , so they should only be opened if you are expecting the message. Many people do not have any software they can use to read Word files, and it is often more of a hassle to read a Word file for those that do have readers. Many people avoid the use of Word and other Microsoft products because Microsoft has been convicted of illegal anti-competitive behavior and has a bad record on security.
Word files are slow to send and waste space.
The email is smaller and faster, and when the recipient gets it and clicks on it they will always see the latest version. Many other types of attachments also cause problems, including Powerpoint, WordPerfect, etc. I you expect them to edit the files and send them back, you need to find out what software they have available and what they prefer. Feedback on how to improve this document is welcome. E.g. better descriptions of how to determine if your document is in Word format, how to choose a better format, etc.