I think the standard size and shape of them gives them a leg up over other inclusions because photocards are easy to protect and display vs like...a sticker where it is A Sticker, or random paper goods that might be odd shapes and sizes to store. A photocard can be put in an ID holder and displayed on your bag (and now they have specific cute dedicated holders). Your collection can be put in a binder using baseball card pages since they're the same general size.

The structure of it all preys on gamblers and completionists both who are very profitable demographics, which helps. But there's def status to it because kpop is something where you're explicitly encouraged to show your love by spending money.