We noticed this after finding an error in our apache config such that some of our assets were set to be cached longer than we really wanted (30 days), much to our chagrin. It turned out that even though you tell the browser to keep something for a very long time, it most likely won't stay there.

Our general assumption was that it is due to the default cache size of browsers being tiny in comparison to the amount of data that users now consume, coupled with every webpage you visit putting stuff into the cache (rightly so).

I believe the default cache of firefox is still set at 50mb which seems very small.