In message , at 10:24:09 on Wed, 17 Jul 2019,
Roland Perry remarked:
Now you mention it, I did once have a case for a smartphone that had
power bank integrated, which meant having a pass-through connector. But
the battery inside that died very quickly (a matter of months). The fact
they are so rare makes me think there's something systemically wrong
with them.

Is it also perhaps because the backs of phones increasingly have buttons
and camera lenses/flashes and so on - which would tend to get obscured
unless the case had exactly the correct cut-outs in it.

The case has to be designed to fit the size, shape and connectors of the
phone anyway; putting holes in the right place for camera, microphone,
speaker etc isn’t rocket science. Plain cases for every variety of phone
manage it, so do the powered cases.

I just went to Amazon to look, for my LG phone. A choice of 13
products, every single one "Currently unavailable".

And trying eBay, all that gives is replacement fatter batteries plus a
"pregnant" back cover. Of course, that's only a possibility *because*
the phone has a slot-in user-replaceable battery.

My first Android smartphone, an HTC Wildfire from about 2010, got fitted
with such a battery (and pregnant back cover) because otherwise the
battery life was well under a day.
--
Roland Perry