Earlier quoted context omitted.
So thickness and weight are the drivers for this: You can always build a laptop that has the same battery capacity with a detachable battery as you can with a non-detachable. It's just going to be thicker and heavier.
It'll also have a great deal of wasted space, more moving parts, more breakable components, less structural support, additional hardware and software validation requirements, and require some careful engineering to avoid having it fall over backward when you open the screen slightly past vertical (battery cells work nicely as a counterweight). All for a laptop that wouldn't sell as well because people do care about s…
I totally agree that integrated batteries sell better - I just wanted to point out that the driver is "cram the same capacity in less space and weight". It's a tradeoff between the three, and since a lot of people prefer lighter, smaller laptops over serviceability, that's where things go. (Written on a laptop with a non-removable battery).