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.