Because they're all using wordpress/blogger/etc which put it in for them.

olavk · on June 18, 2012

If the developers lets these tools insert `alt=""` per default with all images, then they are not in compliance with the spec.

kalleboo · on June 19, 2012

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".

olavk · on June 20, 2012

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