Firstly because the software company can change the licensing rules. At present Word is not the only application capable of opening DOC file types, there are others, but Microsoft could change that tomorrow if they wished. Microsoft might decide to insist that only Word or another Microsoft product could open a DOC file. They would lose a lot of friends that way, but there is no legal reason why they could not so so if they wished. After all, Microsoft have spent a great deal of money investing in DOC. Microsoft could then hike up the licence fees to make opening a DOC file in the future a very expensive thing to do. Now this won’t really concern you if you are an individual or a small business, but it does worry governments (who are paid to worry about this sort of thing on our behalf). Governments create a lot of records that need to be kept forever, and they do not want to be in a situation where they have to pay a commercial company an ever-increasing amount of money just to be able to read their own documents.

Secondly, Microsoft could change the format. And in fact this is exactly what they have done over the years.