The original reason for in device batteries was to increase the battery capacity. Easily switched out batteries require hard plastic cases and battery doors. Better to use that space for more battery capacity.
Plus with higher capacity batteries you can usually go all day on a single charge.
I think phones were required to have batteries you could not easily replace as you can be tracked at all times. You might think the phone is off but it is not.
I read about a criminal who they tracked and the police knew his location by his phone for days and months. There are apps that create maps for you from your travels.
The phone companies / manufacturers figured people were likely to buy a new phone for several hundred dollars if they could not replace the battery for $10 as before because the back is glued shut.
For do-it-yourselfers and those that might commit a crime you can remove the battery.
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