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.