...logistical nightmare, i'm afraid. Typical sex devices move a sleeve up and down, this is impossible if embedded in a doll, you'd have to design a new system from the ground up that fully enclosed one, yet was still able to move up and down within the structure of the torso, without moving said torso.
Unfortunately, once you do this, you remove an incredibly useful feature from the sex device, that of interchangeable sleeves. This removes the ability for the consumer to source exactly the right sleeves for their own peculiar shaped penis (they're all different, i'm afraid...) for whatever situation they desire. So now you have to design an interchangeable, washable sleeve system for every penis on the planet, and make these compatible with your incredibly complex internal fuck machine, that you've hidden in a torso. Also, you need to vent heat somehow.
Not that it can't be done, however all these considerations boost the price into the 'luxury item' range, i think it would have to be hand made, and configured to each consumer individually.
On the plus side, there is constant traction in the sex doll business to head in the direction of 'the sex doll that fucks you'; there's sucking vaginas, grinding hips, AI chatbot heads, etc. There's no reason not to suppose that a sex doll with a fully integrated set of knee / hip / pelvis / spinal servo systems isn't in development right now somewhere. Once you can get a sex doll to ride you with the finesse of an SR6, then we're cooking. However, this is now a sex doll that fucks you, not a sex doll with a fuck machine hidden inside, so different mission statement fulfilled.
i completely agree that discretely hiding the fuck machine in a torso / hips and legs arrangement / wtvr would be a game changer, more sensual ergonomic styling would be an absolute win, and i do think we're gonna get a quantum leap in fuck machine design at some point, very curious where that's gonna take us! Always nice to be involved with a cutting edge hobby, innit…? xD