They're really not - Glassdoor in my experience has always skewed low, after all it's self-reported. Highly-paid people tend to have more reason to stay quiet and not report these numbers because - as we've seen elsewhere in this thread - it often engenders hostility.

I know personally I was more loose about talking money when I was being paid a lot less than I am now.

The big players like AmaGooFaceSoft are routinely paying $300-400k for mid-level talent (5-10 yrs exp), and more for more senior.

In any case, these numbers ought to be known - much of startup-dom hinges on engineers not knowing what the big players are actually paying. If you can successfully convince someone that Facebook doesn't pay more than $200k, a startup salary suddenly seems more attractive. As someone who spent a lot of time in startups personally, I know my jaw hit the floor when I first found out about how high comp can get in BigTechCo.

And then I jumped ship.

Also worth noting: BigTechCo itself hinges on engineers not knowing what they pay ranges are. They absorb tons of talent coming out of startups and academia who were being paid a pittance, and even if they're willing to pay top dollar for this talent, they surely won't volunteer. All the more reason to not answer the "how much are you making right now" question, ever! Revealing your (comparatively low) startup/academia salary is a good way to make sure the "$300-400k" range immediately gets pulled off the table.