You don't need to use glue to have a thin tablet. You can still have 99% of the battery volume by using 3 teenie screws to keep it fixed. And having the battery be easily replaceable with a new one means the user will quickly make up for that 1% if/when the user decides to recycle the battery.
Except you can't replace that glue with "3 teenie screws", can you?
You need to add structure to attach those screws to, for starters, and in practice a containing structure to contain the battery within, etc etc etc.
Sometimes throwing out the brackets/etc and clean-sheeting a simpler chassis that provides more space for battery (providing more run time) with a bit of adhesive to give you a lighter, stiffer end product.
Pick the right adhesive, and you can remove the battery at need by heating or cooling, depending.