Companies whose primary business is hardware seem to be more likely to keep people around for longer periods of time -- Apple, Intel, Cisco, National Semiconductor back when they were a going concern. Software companies, though, particularly ones in Silicon Valley (or following a Silicon Valley ethos)? As someone who's going to hit their four-year anniversary at their current workplace in a week, I feel like the oddball.
I work on software at a similar company - and it certainly seems to have a significantly higher "employment age" than people seem possible here - I've worked here for 4 years and still one of the "new" ones. Some of that I feel due to a good work/life balance (And decent enough pay :), and a culture I find to be pleasant (I worked with Apple employees for a while, for example, and still don't understand why people put up with the crap I saw when there's a healthy job market).
But I wonder if there's something to be said for the work itself - people have been here for ages and still doing new things - one year you may be working on texture pipelines, the next a deep dive in ray tracing. Sure, they're kinda similar, but I feel it may be more rewarding than redesigning a perfectly good UI for the 5th time, or replacing whatever javascript framework you used to use with the "New Hotness".
I think that comes from misreading stats. If a company has huge growth and hires a bunch of new people, the average lifespan of an employee will obviously go down.
There's many well known Apple engineers (in OS software at least) who've been there for decades.