A 25% bulkier "rugged" iPhone 15 Pro would still fit nicely in most people's pockets. Unlike an encased iPhone plus external battery pack.
Like a lot of men, I carry a phone and wallet in one pocket and my keys in another. I don't typically carry a bag. Not gonna carry a battery pack too.
Also, an external battery pack achieves one thing (extended battery life) but not the other -- still wouldn't be able to easily replace the internal battery once it has aged out.