Because we kept buying thinner and lighter phones, and gluing the battery in makes thinner and lighter phones with better battery life possible. As a convenient side effect glue creates a nice watertight seal that can make devices more water resistant.
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What if the homunculus argues that it, as a new and distinct being, has committed no crimes, and deserves to live? What if it begs for its life as you load it into the teleporter to split it back up into your science officer and annoying morale officer shoot it into the sun?
I imagine it’s a hell of a lot cheaper/simpler/more reliable in manufacturing to use the adhesive over screws, gaskets, and grease. It reduces both the BOM and the number of processes required to ship a unit, and probably fails less often, which means fewer RMAs. Plus it just plain takes up less space, which leaves more room for the battery.
I don’t like it, my favorite phone I’ve ever had was a Galaxy Nexus with a zerolemon battery that was bigger than the phone itself, but I can see why gluing everything together would be an attractive solution for the engineers who design these things.