Marketing and culture can do wonders to one's preference.
> synonym for capability Yes and no. An "app" is usually a set of capabilities, not a single capability. Many of the core *nix utilities for example are specifically designed to have a single capability each, and these are then composed for complex requirements. But for the most part today's apps contain multiple, usually overlapping capabilities. For example Microsoft Word offers not only text editing and formatting, but also has a dictionary+thesaurus, spelling+grammar check, mail merge and lots more. Of course, you can abstract this all away as "word processor capability", but there are other word processing apps out there with varying capabilities, and so things become problematic fast especially in terms of interoperability.
> OpenDoc and OLE These "never worked" due to people/policy, not due to tech ability. OpenDoc essentially stagnated at Apple due to conflicts and lack of clarity, and then was outright killed by Steve Jobs[0]. OLE(tm) hasn't failed per se, but it's proprietary to Microsoft[1] and thus doesn't have much adoption outside Windows.
> Brand names matter All they really do is put a label on a bundle - or bundles - of capabilities to create handy associations. YouTube is associated primarily with accessing videos that can be made and uploaded by anyone, secondarily allows commenting on and liking said videos, etc. Again, marketing is a significant factor.
> a built-in feature You seem to be missing the forest for a tree here. That's a single example, of infinite permutations. Also, even given that example you're still missing parts of the flow.
[0] https://en.wikipedia.org/wiki/OpenDoc [1] https://en.wikipedia.org/wiki/Object_Linking_and_Embedding
