There are many reasons for proprietary formats. Proprietary formats are often developed before standards based formats, and inertia is often reason enough for the proprietary format to continue being used: users are already familiar with the format, and companies may not want to invest the time and money into developing new software. Proprietary formats also give the vendor more control. This is important on several fronts. The software supporting the proprietary format doesn't have to be as robust since people are usually using a common set of tools to generate and render the files. (Consider how different software renders ePubs differently. Consider how different web browsers render web pages differently, particularly in the early days of the web. And so forth.) It is also possible to unilaterally adapt proprietary formats, which is desirable while adding new features to differentiate your product from the competitor's product.

On the whole, I agree anti-competitive practices is a major reason behind proprietary formats. On the other hand, the issue is much more complex than that.