Is it not obvious? There's a business equation being made that keeping the batteries internal encourages new device purchases.
Replacing the battery in an iPhone costs $99 and takes hours to days of time where your phone is unavailable.
Or you can just trade it in and pay nothing while your carrier subsidizes it and bills you monthly with 0% interest.
The customer is given an incentive structure that leads them to the maximum profit route…obviously that’s what businesses want, and why every industry has regulations that constrains business to live inside some basic rules.
It’s not like a supersonic jet or moon lander. All phones had removable batteries before the iPhone. All laptops used to have removable batteries.
Older devices built with worse technology had things we don’t have now.