Because everybody wants to be special. Size, weight, optimalization, screen edge corner angle, etc. It is a senseless, unsustainable development practice that creates much waste each year. Nobody seems to look at the future, everybody just wants to win in the here and now.

4 degrees later, we are all gone. The batteries remain, for a while. In 2135 CE, the last Samsung battery too will catch fire, starting an unstoppable wildfire that will consume much of Eurasia, along with whoever survived the previous social collapse. Then, only ashes will remain.

Fuck the pandas, save ourselves.

That was before flagship phones needed to become thinner and bigger every few months. Can't have commonality of parts if your six months old battery is thicker than the whole new phone.

(There's still plenty of phones with replaceable, more-or-less standardized batteries available… but it's $50-200 phones that nobody wants to buy because they're not sexy enough. Even though they've been fast enough for years…)

Custom batteries mean you can make optimal use of space within the device (i.e. pack in more battery life in a smaller form factor).