You say "they couldn't collaborate" I say "Google really wanted to compete".
Remember that it was Google who decided to perform a fork because they had a different approach than Apple for how "multi-process" should be implemented. Google had an implementation that they didn't want to integrate into the Webkit branch - hence the Blink fork.[1]
Also Apple would have been happy if they could use the Google's implementation instead of using their own - they said that repeatedly [2]
You can also find more details of the story here. It looks more and more obvious that the decision to create a WebKit fork was more political than technical : https://news.ycombinator.com/item?id=5490242
[1] https://www.theguardian.com/technology/2013/apr/05/blink-goo...
All I'm saying is that Apple's WebKit is not a good example of OSS and if that's what you have to show for iOS, then it's a really sad state of affairs.
I'm glad that they ended up releasing it, but to show that as an example of open source from Apple ... indeed shows that people have short memories ;-)