As @prototype6143 said, this is done to protect both sides of the transaction. Epic is merely the shopholder here, whilst the content is made almost entirely by other devs like yourself.
Without knowing the specific pack, it’s hard to really comment the validity of requesting a refund, but if the pack is not labelled as working with VR in the marketplace page, then the seller has done nothing wrong here and that would be why Epic aren’t issuing a refund. It’s just an unfortunate result of there not really being a ‘VR’ checkbox when we select platforms. If the seller had clearly stated it works with VR platforms, then you would get a refund more or less instantly, but it’s important to remember you aren’t dealing with AAA companies or anything here, you are dealing with other indie developers.
I’m sure if you contact the content creator, they will at least attempt to help you resolve your (if they are able - sadly most of us don’t have VR headsets yet haha). If not, then someone in this lovely community should be able to point you in the right direction 