It's to do with international regulations around "Fixed" and "Replaceable" battery packs which haven't changed much since Li-ion batteries first were used in devices.

With Fixed the rules about how thick the container the battery is in is a lot less restrictive, Also you don't have to worry too much about sparks and shorts with fixed batteries.

With "removable" batteries you have to have certain thickness of container, space around the pack to insulate it from the rest of the device/vehicle, also access door has to conform to certain standards. As well as precautions to stop sparks and shorting when replacing batteries,. all of which adds weight for not much in return.

BMW did have a "battery replacement concept" :
Where you drive the BMW EV into a bay. The base then open up and a robotic grabber unbolts the battery pack from the bottom of the vehicle and takes it to an underground charging area and another freshly charged "pack" is then installed in it's place.

So you can either buy batteries with the car for an added upfront payment and charge them at home like normal.

Or get a "subscription style" yearly package, where the vehicle is cheaper to buy initially, but you don't "own" the batteries in the car, and the entire battery pack is replaced with a fresh/nearly fresh set of batteries every time you need to recharge.