It's also a lot about the marketability of your skills. The only reason the OP can boast about the experience with these proprietary tools is because they're u

It's also a lot about the marketability of your skills. The only reason the OP can boast about the experience with these proprietary tools is because they're used by FAANG. No other reason — it's just an indirect and inconspicuous boasting about having worked at FAANG. If it was yet another proprietary non-OSS language used by one of the investment companies that has no direct analogues at any of their competitors — good luck boasting on your resume that you have skills in programming languages that noone cares about. Proprietary software sucks. Just because it's written and used internally by FAANG doesn't make it suck any less. When you're evaluating whether or not to join a company, presence of such tools should be more of a red flag, if anything. Same goes with software-as-a-service. Tools should be OSS, and easily portable between the workplaces. Another case in point — it kind of works both ways — many companies start around with Erlang and ejabberd, because it's a really great tool, but then, after having issues finding developers who can understand it properly, they eventually, one way or another, convert their stack back to the more common languages; I recall this has happened with Facebook Messenger and Heroku, and also at a random startup I was interviewing with during the course of my interview (they just hired a manager who knew Java — and, what do you know, he decides to set a direction to eventually replace all Erlang with Java).