Yes that's the problem: emotional investment in their own code. But still the idea of open source is that everyone is free to use everything so that if someone sees something done better they can incorporate it into their own project right? They don't have to copy and paste stuff, but they can take the concept, idea, etc. If they don't and can't get over their investment, then they are doomed to failure eventually because those who do take the best stuff out there and build on top of it will have a technical advantage.

So Firefox and safari are publicly contributed to, but they have different engines. Engines are probably the most difficult part of the browser right? It's essentially the definition of the browser. Well let's say the V8 engine is superior and Firefox and safari don't adopt it. Well they're operating not much too different from a proprietary source. Open source and commercial will both be invested in their products, the difference is that open source can pull from each other, and they should. So if V8 requires major code changes why not? And if Mozilla thinks it can improve it then they can fork it. But Mozilla probably won't do that. You're right most people are invested in their projects and so what happens in the open source world is that brand new projects get started instead, but I'm just saying it also makes sense to ditch when you known you've been beaten just because you can.