And how many different variations of Batman's bat symbol and cowl have been in the comics and films? What, like 50? Maybe more with different combinations. When you walked out of a Batman or Superman movie was your experience impacted more by a shitty nonsensical plot or a comic accurate costume?
With a costume, you could literally show a costume designer a page from a comic and they could replicate it. It might not look good on film or be comfortable for actor, but it could be done. Mary-Jane Watson doesn't exist. You could do an open casting call for red heads in Hollywood, or you could just cast someone you feel is a good fit for the character and make concessions on them looking exactly like they did in the comic. The biggest problem with fan castings is they just want someone that looks like a character. Oh, he has grey hair. He's perfect. So what's more important, the creative team casting someone they think is right for the role as it is written, or getting a high budget cosplay?