What makes playing cards a tougher type of ephemera to collect is that they're made of a material, paper, that's very temporary in nature, and are generally very heavily used and abused by the majority of the people buying them. We buy them, we play with them, they get dirty and ragged from use, then get discarded and replaced with a new pack. It's less likely to occur to a modern custom deck, but only because most of the people buying them are aware of their collectibility in the first place and treat them with greater care.
The same was true of comic books when issues like Action Comics #1 and Detective Comics #27 were new - no one thought of them as something to collect, just something to use and discard, and the fact that they were the first comic book appearances of Superman and Batman respectively never came to mind. Complicating matters, early and middle industrial-age papers were often made using acids that caused the paper itself to decompose more rapidly than modern ones.
I loved playing cards as a kid, but even then only thought of them as disposable, not collectible. It wasn't until I started getting into custom and vintage decks that I took a different view of them, and thus a hobby was born.
Bottle caps, while also ephemera, were made of sturdier stuff (usually steel or aluminum, and only in the last few decades, plastic) and even more common then playing cards, so there's more of them around and there's more people engaged in collecting them. Some paper has a half-life of about two weeks, while the plastics in modern bottle caps would last for centuries, if not longer. Playing card paper is likely a little better protected with its laminate surface, but even so, they're quite fragile when you think about it.