Because the integrated battery adds an expiration date to a device that could otherwise last decades if maintained properly.
Same as Apple tightly coupling the iMac screen with the Mac's software support cycle even though nothing would stop them from just adding, say, a USB-C port that can act as video input.
"Only" $43 billion in revenue is more than 95% of corporations achieved.
Apple is pretending to be eco-friendly and using that as excuse to ship fewer chargers, for example. If they can optimize the same Mac's packaging to use paper that's folded in all kinds of fancy ways, they can add a tiny bit of functionality to an existing port. You can't tell me Apple isn't able to care about small details, because they absolutely do when they want and not only when it's about revenue.