When Barbie first came out, in the '50's her figure was closer to what was fashionable at the time, actual breasts and hips, and a cinched-in waist. Look at the big stars of the time, for comparison purposes. Now, well, that 'model' that was elongated into Barbie proportions couldn't be a 'model'. Too chunky.
And, as noted, some of the design is for technical fashion-doll reasons. It's hard to scale and stitch doll clothes properly, by the time you add a waistband, that tiny waist is filling in. The weirdly long neck looks good in a collar, or a high neckline, while a proportional neck would look jammed-up. The long legs look good in pants, and you can have 'short' skirts (judging by 'height at knee') without offending parents. And the absurdly small feet make sliding on those pants, tights, etc, so much easier. Try it with a flat-footed doll sometime.