I can't say for certain why, but GlassDoor is significantly lower than reality for positions in my area (Boston, MA.) Of the jobs I see advertised for Senior developers, most of them are in the $140-$170K range these days. Assuming that glassdoor is accurate, perhaps the issue is that people who stay in positions more than a few years fall behind new hires because yearly raises don't keep up with market rates.

That and as you get older with more responsibilities many people don't jump ship as often for higher wages. And the higher wage thing is a fairly recent (5-10 year) thing. We can thank Millenials and Vic investment pushing salaries up from bottom. That being said, if you want the money, try to find a 10x company winner take all in growth mode. Life is otherwise too short. Finance also pays well.