> if they're anything like other tech companies, they're doing a shit job of hiring
Facebook grew from 17K employees in 2016 to 45K in 2019. 38% annual growth, and the vast bulk of that was in engineering. "Only" 17% at Google, but it was already larger so the absolute numbers are higher. 22% at Netflix. This is hardly a "shit job of hiring" as you say. I often saw such growth numbers at startups, but never at companies with headcounts already in the thousands let alone tens of thousands. They're doing a fantastic job at hiring.
Part of the reason they're hiring so much is because they have to. Again contrary to what you say, they are not holding on tight to the good ones. The number of people who leave after two years - at all levels - is almost as stunning as the number coming in. Big Tech is notorious for this. The good ones might leave to found or join startups, but leave they still do. The high compensation actually facilitates such departures as much as it discourages them (certainly did for recently-retired me), except among the exceptionally greedy.
These companies are not pools of carefully selected and tended talent, with low rates coming in or going out. They're more like massive pipelines, with huge numbers entering and leaving all the time. The coding interviews are a very coarse filter to keep out the worst of the dreck, but they also keep out many good engineers. I've known several demonstrably excellent engineers who failed in that moment, and more who self-selected out of the process for various reasons. The people who can get in are barely distinguishable from those who got jobs elsewhere via more traditional interview processes, except for skewing very young.