The fact that so much of the web is closed source now is a great tragedy, and is the result of the fact that those companies primarily see themselves not as software vendors, but as service vendors. They see humans as a kind of zoo animal who isn't really capable of engaging deeply with their software products, and can only jab at buttons.
If Google and Facebook weren't run by supremacists, and instead considered the general public as intelligent beings who are capable of understanding software, we might see more software coming out of those walled gardens. But they don't. Facebook and Google "engineers" (and software "engineers" at large) think of software as fundamentally hard, fundamentally technical, fundamentally something for the precious few with brains big enough to engage with it deeply.
It's just like books used to be, when the educated considered reading too advanced for the unwashed masses.
And so these service companies toss apps over the wall for the plebs to play with, and they keep all the cool toys for themselves.
There are some exceptions. They open source some commodity tools, but only where there is open source competition forcing them to do so.
I find it quite sad. But they'll lose when the web comes roaring back, when some IDE learns to operate within the language of the plebs, and the plebs learn to code, the wizards at Facebook and Google will find out what happens when you treat your customers like they are too stupid to operate at your level. Silicon Valley runs on supremacism. Supremacism breeds resentment. Bumpy roads ahead.
Some people at Google seem to understand this. I see them fighting for an alternate worldview. Maybe when the change happens, that minority will gain control of the company.