Having opened both my 2008 Macbook (with removable battery) and my current 2015 Macbook Pro (with non-removable battery), it's easy to see why: the 2015's battery is actually 4 different batteries of varying sizes & shapes. It's taking most of the space inside the computer that isn't taken by the motherboard (about half, actually).

Making stuff removable is a lot more work; you have to build a way to hold the thing while it's on, you have to put in connectors, etc. With non-removable batteries they can just glue them on & solder the wires.

Not really saying it's a good thing, but I understand the decision.

Purism Librem 15 is also pretty thin, but the battery is easily removable.