Your last sentence is the answer to the first.
Inside an organization, Exchange will perfectly happily do secure mail - it works, it's nearly hassle free and novice users can do it.
Outside, you have a pile of Web of Trust problems which novice users don't understand, and no pushing requirement.
I am surprised that large businesses have tolerated this for so long. Maybe SMIME isn't the answer, but it's surprising that we haven't seen some method to guarantee secure delivery of mail between companies.