That's because it's a near-meaningless title that has been watered down. Everyone is a "senior" now by their second year working.
That's why anyone with 5 years on the job is considered "senior", even though most in most professions you're just getting out of the entry level after 5 years.
Most of these companies have solid internship programs, and almost all E3 (entry-level) positions are offered to returning interns.
What this means is that there's no track into a junior position where you can gain experience and work your way up to senior within a FAANG if you're older than a recent college grad. That's unfortunate, because as someone working for a mid-tier company I'd like to work hard and land a senior role at a FAANG but I often wonder if these companies would look down on candidates who have spent years at lesser companies.
I have 15yrs experience and moved sideways a few times and I am in effect treated as if I were new in my current role.
So senior doesn't so much translate to experience or even transferable working knowledge but rather 'how well you have done politically to gain position in this specific company'. Yes I am cynical :)