> We have libraries like SQLite, which is a single .c file that you drag into your project and it immediately does a ton of incredibly useful, non-trivial work for you, while barely increasing your executable's size.

I'm not sure why you believe this is more secure than a package manager. At least with a package manager there is an opportunity for vetting. It's also trivial that it did not increase your executable's size. If your executable depends on it, it increases its effective size.