I think the thickness is just a minor factor, but there are two major reasons why they are out of style:
1. Making the phone water and dust resistant is much easier without replaceable batteries
2. The manufacturers want to sell smartphones and replaceable batteries are probably the single feature which extends the lifetime of a smartphone the most
The first reason alone would be good enough for many consumers to give up on replaceable batteries but combined with the second reason we come to the situation we are currently in.
1. Making the phone water and dust resistant is much easier without replaceable batteries
2. The manufacturers want to sell smartphones and replaceable batteries are probably the single feature which extends the lifetime of a smartphone the most
The first reason alone would be good enough for many consumers to give up on replaceable batteries but combined with the second reason we come to the situation we are currently in.