Honestly, I work at a FAANG and, sadly, the answer is often a combination of people paying absolutely zero attention to tech that's NIH (e.g. SWEs working on DBs who have never used Postgres) and the fact that no one gets promoted for implementing a solution using 3rd party software. The system is setup such that you need to show you've developed something of sufficient complexity, and using OSS just doesn't look good in that context.

>If you're smarter than the hundreds of engineers designing and building the distributed systems powering the world's largest applications then well done.

There is plenty of absolute code abominations powering "world's largest applications"

Engineer competence is also vaguely related to quality of the infrastructure, yes, you need smart people to make big complex things, but you also need smart people to manage ungodly legacy enterprise spaghetti

You're assuming a level of engineering/scientific "purity" that doesn't really exist within orgs to the idealistic extent that we would like to imagine. I'm not saying that I'm smarter than every FAANG engineer, but having met quite a few and worked in large orgs myself, it's easy to say some of these tools come from pride/arrogance/a need to have your name on something/a fundamental misunderstanding of existing tooling. Not necessarily because they're unraveling parts of the universe yet unexplored and need a bespoke weapon to tackle new issues.