Probably because there's no one at Facebook or no infrastructure that is specifically setup for hosting stuff like this. Facebook is a product company, not a hosting company, so this makes perfect sense.

I work in the marketing department of a late stage technology start up and we face the same problem. We have lots of excellent people all over the company, but it's a tradeoff in terms of asking people to work on Wordpress hosting infra vs. just paying a service like Wordpress VIP to handle it.