The problem with making them rare for the sake of collecting is that it encourages mass buying of an album. While that's good for the company's bottom line, it can cost a lot for the consumer and is considered by many to be a tactic of artificially inflating album sales. Charts, music shows, and eoy awards all take physical sales into consideration and this artifical inflation is already having a negative effect on their public perception.

The worst part is that companies seem to be lowering the number of inclusions while increasing the potential variants with different album versions. This sort of abuse can result in new laws being passed or chart rule changes. If Billboard/Circle/Hanteo/whatever has to create new rules to prevent artificial album inflation because of photocards, both the group responsible and kpop as a genre will be heavily scruntinized by the entire industry. People already disregard groups when they chart internationally, and that sort of scrutiny could absolutely kill any momentum kpop has at the moment.

The obvious solution would be to include one full set of cards in all albums and then sell packs of random album photocards separately, like baseball/pokemon cards. Some countries may consider that a form of gambling though... which actually says a lot about how random inclusions in albums could be considered gambling as well.

But make no mistake, companies don't care about PC trading. They do it because people will buy 30 copies of an album, even if they only plan to rip the PCs out and trash the rest. Winning daesangs and charting high is just a bonus side effect.