To make the phones "waterproof" which is funny because even tho phones are water "waterproof" manufactures won't warrenty them out if there's water damage. Doesn't make much sense to me.
With how capable modern mobile CPUs are phones could really last a long time without feeling "so last generation" very fast. So the battery really often is the bottleneck and not having it be replaceable would really just be planned obsolescence with extra steps. It's good to hear some good news once in a while.
It's more expensive to waterproof a phone with a removable battery. Yes, there were phones from the 2010s that were waterproof and had removable batteries, but that was a feature you had to pay extra for.