Whilst not perfectly circular, Mandolin family instruments often have a teardrop shape body, as do Portuguese guitars. No idea if there is any sonic benefit to this, most likely just the traditional shape.
MOST of the original string instruments started off round or close to it, it was simply much easier to build, or a result of using something like a turtle shell or gourd for the back.
The banjo was
invented in Africa, BTW:
This persisted on up through the early middle ages with the lute ( "l'
oud" )
which evolved into the vihuela:
When I was introduced to classical guitar I was told the instrument was given its shape to mimic the outline of the female form, but I have a suspicion the ergonomic advantage of being able to hold the instrument at an angle or play on horseback was the real driving factor.
More precisely that, as some mentioned, being able to hold the neck at an ergonomically advantageous angel without having to use the hands to do it, was found to be highly desirable.
In fact my first classical guitar lesson consisted entirely of an analysis of the ergonomics of the instrument, and how to hold it correctly, and why the fretboard was designed the way it is, nothing else.
It was a mind-expanding moment for a 12-year old, no joke.