Don't confused proprietary and industry-standardized. Yes, pens can inter-operate, but it is not because the inherent nature of a pen is, "open," in a business sense. In fact, some pen designs have patents on them, the essence of proprietary.
What you are talking about are industry-standards, where manufacturers come together and agree upon the best way for them (or at least a specific subset of them) can compete and still make money while not focusing on brand lock-in. As a result you get specific standards, such as lead core sizes or inkwell tube lengths. In the software world, one might make the comparison to open data formats which force software vendors to sell based on product features and usability and support and not just because of data lock-in. Or finally, look at web standards. We have the open Gecko and WebKit engines, but then there is also the closed, proprietary, IE and Opera engines, which all are able to display web-based content in relatively standard and uniform ways.
What you find is that when markets mature, the tools, the long-term products, remain proprietary, while the short term commodity consumables often become standardized.