It is designed to entrap people who don't really think about their tech. I know a few people who are in a sort of "late stage icloud" moment with their iPhone. Their phone started filling up with photos and videos years ago. So rather than transfer to some external storage, as they might not even have a working mental concept of a file system let alone a personal computer that isn't work issued anymore for that matter, they started having to buy bigger and bigger storage capacity iphones. Eventually those got full too, so they had to go to icloud and a few have basically maxed out their icloud 2tb plans, because these phones are now producing huge files and recording in 4k I guess even though you are still just looking at it on a 5 inch diagonal screen like in the 5mp phone era. The damn icloud previews can fill up your phones internal storage even with everything technically shoved off to icloud.

Frankly even if you know what you are doing apple doesn't make this easy. Image Capture is afaik the only real way to transfer photos to an external drive on iphone. And that app has not really seen any love since release, its buggy and frequently crashes with large transfers in my experience. It does weird things like if you select 13 photos to delete, it will delete them but not remove selection having the next 13 photo block pre selected for reasons that are unclear (Maybe the devs know how terrible the transfer experience is so they make it easy for you to throttle the job to like 250 photo chunks with this ui quirk). And what is worse, those icloud previews technically don't exist in the eyes of Image Capture, so those won't be transferred over let alone show up in the image previews.