Mostly space, but inside the device. Laptop batteries were comprised of several cells, and therefore requires a case.
The case would add several mm of z-height (thickness), something that laptop manufacturers are obsessed to shave these days. Then you’ll need to implement how the battery and the rest of the system attaches. You can’t just use the small connector like on sealed-in laptops. See the picture below
Instead you have to use a far more robust, larger, and expensive connector, and you can’t also use a cable like that. Everything has to be designed with robustness and user-handling in mind, instead of handling by a professional service technician.
Finally, the certification is also a bit more complicated with a removable battery since you need two products certified instead of one. Not sure about this CMIIW.
See the CE and FCC logo on the battery.
So its less about weight, but more about physical space inside the device and cheaper R&D/manufacturing in general. Oh, also business side as people are more likely to buy a new laptop once the battery degrades. From the snake oil’s perspective, I don’t see a reason to do removable battery.