Not entirely true. Facebook, Google, Microsoft, Netflix, etc... They all wrote (or greatly contributed to) a bunch of high quality open source projects that you probably use on a daily basis.
Yeah they did. But its a small fraction of engineers that wrote those libraries. Not only that, most of the engineers doing foundational work at those companies are not getting hired through standard interviews. When Google wants to hire people for a team to write a new distributed database, they arent asking a bunch of leetcode questions to engineers. They're hiring out of PhD programs and from accomplished teams across industry. People do get pulled onto those teams from other teams in the company, but it's only after those people have proved themselves. As a whole, only 1% of the engineering at those companies is what you are describing.
Look at the white papers for the foundational systems at Google and FB, most of them are authored by the same few teams. Jeff Dean had a hand in designing a ton of large scale Google systems, just because he came up with some brilliant solution, does not mean Joe programmer at google who can implement A* in 30 mins is in the same league.
