AppAmaGooBookSoft want few things more in life than for their engineers to remain really, really smart at developing distributed databases and really, really stupid at salary negotiation. They want to avoid further repricing of their entire engineering workforces at once, as that costs (literally) billions of dollars. They take substantial measures to avoid this, including (in recent memory) at least two of those companies illegally colluding against their employees to avoid those employees learning accurate facts about market clearing prices for their labor.

This also helps develop cultural norms like e.g. "We're here to organize the world's information. It is Ungoogley to collect a spreadsheet about Googler salaries. That information should never be organized... by you, that is."

It's not actually all that hard to find out. You apply and if you pass, you get a silly offer. It actually does happen. A year ago, I could have written the dismissive, "individual contributor engineers making that kind of money is a pipe dream" comment myself, but in the meantime I happen to have interviewed at one of the bigs and more than doubled my take-home overnight. And I was reasonably well compensated before.