Let me add some perspective to this as both an engineer and a Google employee.
The first thing I'll say is this: people lie about their incomes. They even do this anonymously (Glassdoor, etc). Not that I'm calling (or even suggesting) this a lie but be very careful believing anyone's salary claims unless they show you an original offer letter, employment contract or a W-2.
I'm not even sure if they're simply bad at math, lying to themselves or lying to others. Whatever the case, they lie.
With respect to Glassdoor and similar sites, another problem is different people have different ideas of what "salary" means. Does it include stock? Actual bonuses? Expected bonuses?
Likewise it's a skewed sample. I kinda have my doubts that many principal/distinguished engineers have the inclination to accurately report their incomes on such sites.
That being said, my experience in the outside world is that your salary quickly tops out as a senior engineer, architect, whatever. The only way to increase it is to move into management.
While you can move into management at Google, you can go very far (in terms of career and compensation) being simply an engineer if you're good at what you do, get things done and have a lot of impact.
This compensation can take many forms (vested stock, base salary, annual and periodic bonuses, etc).
Also, Google realizes that after you've been here awhile you become increasingly valuable. This is true for an engineer no matter where you work. The cost to a company of replacing someone who has worked there for years is huge (both in recruitment costs and getting them up to speed). Google is simply the only company I personally have worked for that seems to both recognize this and build it into the compensation system.
This really is a great place to work. That alone attracts and retains an awful lot of people. You can also get to work on some very large problems and systems (another draw card). The fact that Google does (or can) pay you very well is just icing on the cake.