Today I read this great analogy that inspired the title of this entry. Why is this so? Once you enter the theater you are now part of a captive audience and are expected to part with your hard earned cash. Even in the days of BYOC (bring your own candy) to the movies you don't dare bring your own popcorn.
This is an example of how proprietary SW vendors get customers apart of their ecosystem and get them locked in. In the end it's a package deal. You can bring your own APIs and some tooling but the vast majority of the proprietary SW will be sold in the millions of dollars to customers who have bought their ticket at a theater near you.