It's not about philosophy or religion. There are clear benefits to having the source available, not the least of which being the fact that you can learn from the code.
How you assess the balance of benefits versus costs of open source is a question which reaches into your philosophical beliefs. As for the distinctions between "philosophy" and "religion", well...that's a well-trodden path I won't go down.
Open source is not free. It's charitable work, which might be purely altruistic or might be largely self-interested, or anywhere in between.
For what it's worth, I do believe that the Swift frontend "should" be open sourced. I believe that I would benefit from that happening. I also believe that Apple would benefit, but I won't pretend that it wouldn't come with costs to Apple.
Apple has decided, at the present time, that the costs outweigh the benefits. Their philosophical stance on open source does not persuade them to incur the costs, at this time -- and possibly never will, though we both hope otherwise.
There are dubious benefits. I use Emacs all the time, and have made a few contributions to it, but I won't be digging into the core C code anytime soon. First, learning enough to fix even a simple problem takes quite a bit of time. Second, the lessons learned will be mostly about Emacs, and not generally applicable; that's the way things are with most big programs.
The amount of time required to meaningfully learn from or contribute to something as hairy as a web browser or compiler is large enough that you can only engage with a few. Personally, I never want to have to look at my web browser's source code.
Would you prefer "philosophy"?