Well, Seth, it has to do with human nature, and a very real responsibility to Sellers.
The human nature is, listing agents do not serve the general public, they serve their seller. So if you have to call from outside a property, the listing agent doesn't much care about your inconvenience. That's one thing.
Secondly. There are various stages of being "under contract," and MLSes have specific rules about how to categorize them. Generally, as layers of contingencies are satisfied, the stages change.
Here in Sunny Seattle, we may be "under contract," but none of the buyer's contingencies have been satisfied. Pending Inspection is an MLS category, and listing agents will not put a notice up on the yard sign, because, well, it's still worth having Seth call in on the property.
The financing contingency is one that isn't totally within the buyer's control, but when that's the only remaining contingency, then we'll call it, "Pending" and the buyer's agent can put a "Sold" strip up on the sign. The logic here is that the buyer's satisfied, and it's now pretty much up to third party approval.
That's the story.