Because when most people use the one email host, they neglect all other users. Even my yahoo email is regarded as second grade citizen now. A hospital straight told me they refuse all yahoo addresses.
I mean, that's certainly retarded, but most businesses aren't that stupid in how they choose to run things. I've never, not even once, run into a business who cares what your email is. They will use any address you want, whether you have a known provider, or self host.