That's true, but most companies need to spend at least some of their time on writing their own code and features too. So we pick third party code that serves us

That's true, but most companies need to spend at least some of their time on writing their own code and features too. So we pick third party code that serves us and saves us time. Analogy if the goal is to go for a smooth car drive, why would you want to stop and be replacing/reinventing wheels every few minutes?