Because, as far as Apple and Google goes, I have researched many of their libraries including their cloud computing software for years. They may have their own tooling but when you understand the sheer amount of what you have to know to put all things together it is kind of mind blowing. I have also watched many universities and professors too. No one knows it all but you can specialize.
Here is something to think about. You guys consider CM to be a security expert yet this site took about 150 developers 7k commits over 3 years to build. And they still used other people's software for basic table layouts and used canned apache search engines. Could you imagine what goes into building a security library?