I wasn't ever a part of a creative team, but as a fiction writer I feel really inadequate compared to the artists. 'A picture is worth a thousand words' is painfully accurate when you measure your progress with a word count. The impression that a single artwork delivers much more than two pages of my writing is very tangible.
Then there's what @emkay mentioned. A picture is simply more powerful and easily digestible, and that translates into reception. I may just add that there's a great difference in effort, too. Text requires at least some conscious processing (not the individual words, though). Images are processed automatically, almost instantly and, in a huge part, below our sensory treshold. Reading takes work; digesting a picture happens almost without effort. Most people will pick a picture over text because it's simpler and more rewarding.
While I wouldn't say I'm jealous, it's still very disheartening. I feel artists work with a superior medium that makes their work much more effective. I don't begrudge them - I know it's infinitely tougher to learn art than writing - but when I compare the reception they get with my own, I feel kind of hopeless. It's so easy to ignore or dismiss my work that I have to struggle just to make readers give me a chance. To artists, it comes naturally. It's hard not to do at it and think 'I'm doing something wrong... Then realise that it's writing.
In fact, one of the most common pieces of advice writers get to make their stories noticed is 'you need a good cover art'. I don't think there's a better way to express that gap