Sorry for the list reply, forgot to disable digest before posting.
> There isn't really a "history" to this, and there certainly wasn't a
> design decision. It's just how it is. Shortcodes weren't introduced
> until version 2.5, four years after version 1.0.
> Along with root-relative URLs, this gets brought up from time to time,
> for example https://core.trac.wordpress.org/ticket/11394. There's not
> much benefit to an img shortcode over an img tag, and there are
> downsides such as (as you pointed out) additional database queries.
Thanks for the track link.
I disagree about the potential benefits. First, if a theme change is done and the sizes for medium or large change, all images in past posts would be automatically updated after regeneration, all else being equal. Huge timesaver when required.
Second, when the responsive img stuff like srcset becomes a thing, it would be very easy to apply it retroactively, assuming the shortcode also has a hook.