Only up to a point. There is a ton of scenarios where you simply run up to the API limitations.
Why haven't people bludgeoned them into opening their software if its so useful? This is inevitable; in fact you could measure the long-term efficiency of an industry by how quickly this happens. Movie studios are famously bad at spending money (yes there are exceptions, but they can be counted on one hand).
Because most people are much more concerned about doing their jobs than having access to source code they aren’t going to look at, much less modify.