Remember that empty alt texts (ie.
<img alt="" src="image.png" />) are acceptable if the alternative content is repeated immediately before or after the image, or if the image does not serve any important semantic purposes.
The most frustrating thing, however, is that by default the filler 'alt text' is filled in the alt text blank for images, and more than 90% of the time this is left in the post after submitting. This is not to fault the post authors - coming up with a valid textual alternative to images is hard, and sometimes as notes above sometimes there's no need for a textual alternative for the images.
That said, having no alt text at all would arguably be better than having half a dozen 'alt text' images strewn across posts. The editor should, if possible, remove the filler text automatically when the post is submitted.