A lot of them get there by being interviewed by other bad engineers, and just float when the market is strong, or use politics to get in a good position.
A guy at my company just merges library upgrade PRs blindly. That's all he does. He fought getting dependabot up on Github, but he finally conceded. He hasn't done a commit since. I've reported it to management, but I was told it wasn't our team's responsibility.
There are tons of dark corners to hide in at big companies.
I don't know you personally, and I don't want to start a flame war here, but if there's one thing FAANG senior engineers are good at, it's solving simple coding problems in interviews
If you're saying they can't actually proactively get useful work done on their own, or quickly ship quality products, or talk to customers, I could understand. But basically the only thing that you can be 100% sure of is that almost everyone at those big companies passed some arbitrary coding tests
At some point, a faang engineer was good at solving those, but that might not be their current state
You generally only get good at interview problems when you're entering the door. After that, interview questions generally have nothing to do with your job.
Also, as you move beyond entry level positions, political skills, communications skills and connections becomes more and more important. Unless you happen to be at some massive growth teams at the right times, it's basically impossible to move beyond ICT4 or L5 unless both you and your boss have very good grasp of politics.
5 years out of college, worked in a single place all those years, without ever touching data structures or algorithms under the hood? Yes, I can absolutely see even experienced programmers stumble on relatively "easy" questions.
But that's just the gatekeeper to get them inside the door. You could be a good engineer without knowing how to find the optimal solution to some niche problem, on the spot.
One time the candidate had a bunch of video games out in plain view (Switch and Xbox, IIRC). Apparently couldn't be bothered to put them away for the remote interview. They proceeded to flail and scored a zero on every question. What are people thinking in these scenarios? -_-
For one thing, if by your own analysis, these companies' interviewing processes aren't effective, there's logically a conclusion that yours also aren't, given that interview processes are fairly similar among big tech companies. I've even also heard of an interviewer trying to tell a director with a straight face that a high level IC candidate (L6/L7 IIRC) from a prestigious big tech couldn't solve a "simple" problem at one of those companies. Leetcode style questions aren't even necessarily good proxies for whether an engineer is actually effective at their job or not (and yes, a good number of FAANGers openly admit here and elsewhere that they needed to grind leetcode to pass interviews at these companies).
Another thing is that Uber and Amazon are fairly competitive w/ other more prestigious FAANGs in terms of compensation. A decent staff eng at Uber isn't going to be very interested in grinding leetcode for a equivalent role at Google, the compensation delta is just not appealing[0]
[0] https://www.levels.fyi/?compare=Uber,Google&track=Software%2...
That's probably why you're interviewing them. I doubt the ones who are good at their job are leaving as much as the ones who aren't. This is like reverse survivorship bias.