With early smart phones generation there was basically a race to thinner smartphones. Replaceable batteries need a protective shell so it wouldn’t get damaged easily when idiots fumble with it when they replace the battery. But the protective shell takes up space. So the first thing they did to make phones thinner was to remove the replaceable battery and just use a battery without a protective shell. Also Apple proofed that most people don’t care so all the Android phone makers followed suit.