I'm not even sure thinness was something consumers ever would have demanded (at the sacrifice of battery life) if the mfrs hadn't pushed it as a selling point.

In the flipphone days I didn't know many people who didn't have at least one spare battery, so they could swap to a fresh one on the go without having to charge, or bought extra thick batteries with higher capacity, extending the back of the phone.

Then when smartphones had removable batteries, lots of people still did those things. And all during that time I remember many reviewers and consumers reacting to many of the "thinness" claims with "I'd really like a bigger battery instead."

I also remember it being proven that apple's removal of the headphone jack impacted neither waterproofing nor thinness, despite their claims. (But then of course one by one others started following suit.)

I think it's better for mfrs and that's the only reason. It saves them money on mfr, or gets phones tossed in the bin faster. Possibly both.

I'd still take 2 or 3 more mm of thickness for an amazing battery.