[Czarcasm]

Rubber alone would require pressure pushing the battery against the housing to maintain the battery position during a drop. This pressure against the underside of the display module would mess with the display.

Adhesive is needed because there can’t be any pressure against the underside of the display module, and the battery can’t be allowed to move even a slight amount during drop impacts.

[strunz]

Also batteries under pressure is a dangerous and bad idea

[cubefox]

If this was true, phones with replaceable batteries would be impossible.

[londons_explore]

You can still put the battery in a box, and make it true inside the box.

[mschuster91]

> If this was true, phones with replaceable batteries would be impossible.

They're thicker to account for a solid plastics barrier between the battery compartment and the display backside to protect the display from the user during battery replacement. Glueing the battery to the phone backplate allows the manufacturer to skip the .5mm of plastic.

[brookst]

Not impossible, just less durable. See also: sockets instead of soldered chips, connectors instead of soldered wires.