we keep kosher so we have to shop in a kosher. store. Non Jewish stores have to be competitive since you would go elsewhere. Jewish stores know we have to come there so they rip us off on the other items as well
When you walk into Walmart or any other major retailer, they have a huge selection of candies and drinks by the checkout, all priced well above what you can get them for elsewhere, even in their own store. It's called convenience.
Personally, I don't think I've purchased fresh produce in a kosher store more than 5 times in my life, excluding special yomtov needs.
ETA: And again, it's not ripping you off, even for the convenience. The margins on produce are not astronomical, once you factor in higher wholesale costs and waste.