And requiring alt means you lose that signal, since content providers will continue to be lazy and leave the alt blank, so now all images look like they're decorative, instead of some being "we don't know".

You can't solve a social problem (content providers who don't care) with a technical limitation (required alt tags).

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".

No, it is correct according to the HTML5 spec to leave out the alt-attribute if there is no valid alternative text available.