It is understandable that you would like to get a refund for an item that does not fit your project needs, just keep in mind that epic have to look after both the buyer and the seller here. The marketplace is more or less a platform maintained and moderated by epic, most of the content is developed by people outside of that, which is why the refund policy is in place.
Buying and item means that you as a buyer have agreed to purchase a product which you have evaluated prior to the purchase and would come to your own conclusion whether it suits your own project. If you are unsure, then its the sellers responsibility to answer or inform you whether it could meet your requirements. If the product does not meet your needs based off information, descriptions, marketing content, sellers response, etc, then you have more then a right to make ask for a refund and state why and where it fell short.
From the sellers perspective, they are not selling a demo (maybe some do in other shapes or forms 
) and if everyone purchased then asked for a refund it would break the integrity of the marketplace. Like pbarnhardt mentioned, protecting the content from people who can (and probably have) bought the asset then copied, distributed, then asked for a refund on top of all that can be very challenging which is one of the reasons they need to be strict with how the policy works, for everyone.
Making an exception here and there would not ground Epics policy and would even turn developers off creating marketplace content up if users will constantly ask for a refund. The policy isn’t really new or that special. If you go to a grocery shop and buy some food, then get home, open it then realize you dont like the taste, then its unlikely you will be able to get a refund. If you open it up and its moldy, then you have a much higher to get a replacement or a refund.
The difference here is between the quality of the product vs the product information given to the buyer.
Most of the content is also produced by indie developers, and we all defiantly dont want to buy anything that doesn’t meet our needs. What is given to you is mostly on the sellers side, but Epic’s marketplace is a means for us all to distribute, share, and support content within the community, like many other platforms with similar policies in place.