It's about trapping the customer
I think that you'll find that, even if an open standard can be used perfectly well for their use case, companies large enough to fund their own private alternative will always do so, because their customers become trapped.
There tons of examples of it throughout recent history, as some have mentioned already. I know many people that held onto bad ISP services for far too long because they couldn't bear the idea of changing their email address. We watched Google kill their standard messaging protocol and replace it with their own, while adding virtually zero functionality gains while simultaneously requiring a decidedly crappy application to use it. We saw Microsoft do something similar with Skype, locking out the possibility of 3rd-party compatibility. We witnessed the web slowly grow into an abominable, shambling horror made of Macromedia (Adobe) Flash, which persists to this day despite superior open standards in place that make Flash look like a cruel joke. Many, if not most, of these decisions had a lot more to do with locking the customer out of alternatives than it had to do with improving the user experience, functionality, or overhead.
I am sure that if I stopped to think about it for even 5 minutes, I could think of at least a dozen more clear examples of this same misguided (or possibly sinister) practice.
Fortunately, I think there is still reason to hold out hope. Even the current masters of the universe in the tech and communications worlds are still relatively new in the grand scheme of things, and we've seen similarly large titans (who also seemed unbreakable in their heyday) slowly kill themselves with bureaucracy and resting on their laurels. The only thing that is certain is that nothing is certain.