A lot of people using a lot of fonts from a lot of different browsers. I'm sure the caching techniques aren't perfect but they're probably quite good.

It's been a while now, but at one point there were some numbers that came in that showed that a much higher fraction of user agents arrive at a web site with a completely empty cache [than you might expect].

Enough so that if you rely on caching for user experience, your average user is not going to have a good time.