I think there’s a couple of market motivations to not make phones with replaceable batteries.

The first, most enticing is definitely gating repairs. Now that right-to-repair laws are picking up steam, that one is starting to fall apart.

The second one is minification, they want to make smaller phones, more lightweight phones, and unibody constructions make that goal easier. You have to incorporate seals and locking mechanisms on a battery door if you want to achieve the same level of water-resistance current unibody phones have. This one is also flimsy, they could design a smaller phone with these features if they wanted to put money into R&D for it, but they likely currently don’t care enough.

The last one is brand integrity. Apple specifically has a design language that many customers are familiar with, if they drastically change and break apart their design with big changes it could have an impact on how customers view the company. This one is probably the most ambiguous, I’m sure Apple has determined over 20 years the level of impact changing designs too much in a generation can have on their bottom line, they likely wouldn’t suffer much. Until this point there hasn’t been much motivation for them to do this.