Obviously there’s no such thing as artwork being objectively right or wrong. But if the intention of an RPG cover is to interest the viewer and/or give them an idea what the product is about, then yes, they mostly fail.
I had a look at Amazon’s current listing of top-selling fantasy books today. There seem to be two main styles for the covers.
Obviously some RPGs are going to deliberately try for a comic book style. But this is the 4th edition Dungeon Master’s Guide 2:
It’s not a budget or skill issue either. I’m pretty confident that a ‘single symbol and text’ cover would be easier to do, or cheaper to commission, than full-page artwork of a PC vs monster skirmish. And there are lots of stock photos out there if you wanted to do the other main style.