If a content provider don't care about accessibility, why would they care about validation? They can leave out the alt-attribute whether it is required by the spec or not.
Because they're all using wordpress/blogger/etc which put it in for them.
If the developers lets these tools insert `alt=""` per default with all images, then they are not in compliance with the spec.
But if they leave out alt="", then they won't validate. What are they supposed to do, force the user to enter text? Then you'll end up with alt="sldfjsjf".
You can't solve a social problem (content providers who don't care) with a technical limitation (required alt tags).