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".
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.