Each doll is made of 15+ individually cast pieces (at minimum - two feet, two calves, two thighs, two hands, two forearms, two upper arms, torso, head and headcap, and that is for a single-jointed doll with a one-piece torso, many dolls will also have extra knee, elbow, wrist, and ankle pieces, and the torso may be in two or three pieces instead of one).
Those pieces then have to be hand-finished (removal of extra flashing material, sometimes elastic channels drilled for the stringing if that was not already included in the mold, sometimes full seam sanding), and assembled with elastic and metal hooks.
The silicone molds are only good for about 10-20 castings before they start having enough wear and tear to reduce the quality of the resulting doll, so a lot of the material cost is actually in the silicone (platinum-cure silicone is much more expensive than resin). New molds then must be made off of the original master sculpture.
Then, if you bought the doll with a face up, an artist hand paints the doll's features with pastels and acrylic paints. And if you bought it blank, you will have to either paint the face yourself or commission someone to do so eventually, as blank dolls really don't have anything in the way of features - imagine a 3d model with no textures applied.
So that's just the incremental cost of each individual doll. On top of that you have to pay for artist development time of new sculpts, marketing and websites, translators if you want to be able to sell your dolls worldwide, all other other usual overhead costs of running any sort of business, etc.
Edit: Also, I don't know if most people really realize the size of some of these dolls. The ones I have that start broaching the $1k mark are anywhere from 65-72cm tall, and weigh as much as 10 lbs. One of mine is 80cm tall, meant to represent a 12ish year old girl in 1:2 scale. She's so big I buy her shoes from the toddler department, not a doll shop.
In the past everything was expensive. The world of mass production hides from us how much it costs to make 1 of something. A few hundred years ago books were insanely expensive and almost nobody had them. A hundred years ago toys were hand made and expensive and a child might have one or two, most likely passed down from someone else.
We live in a world where the amount of cheap stuff has exploded beyond the imaginations of anyone from the past.