Facebook also shows what should/could be the whole page, but I'm not sure we know what all they strip out in the name of making things faster or more pleasant. I'd wager the same is true for any built-in browser that overrides the default action of sending a web link to the actual mobile browser.
And I've gotta say, I'm really bothered by the way Facebook handles browsing. I have to copy the link, exit Facebook, open Safari, then paste and navigate to the link whenever I want to have a page I found on a Facebook post in my bookmarks, reading list, etc. Super frustrating.
I mean when I click the link in a post, or click a link post. If there's a hidden option somewhere that's something, I guess. But I'm going to bet most folks aren't going to go hunting for the hidden option...
You can make it automatically open links in an external browser (on android at least?). Hit the little hamburger icon on the right, scroll all the way down to app settings, then turn on 'Links open externally'
On iOS, yes, it has a built-in browser. You can go to Safari if you like.
They're not packaging up web content as if it's not web content, though. It's just to access sites quickly.