My chickens love fresh vegetables - lettuce, cabbage(chopped into fine pieces), mustard greens, bok choy, choy sum - they can go through 2kg-3kg of green leaved veges amongst 6 of them in a day if I let them.
On top of that, they free range most of the days on the lawn, they also nibble on the grass whenever they feel like it.
From observation of my own chickens, the possible reasons why they don't eat the veges that you have given them:
1) It's too hard for them to bite into, i.e whole pumpkin or apple
2) Food needs to be introduced to your chickens, ideally when they are young. Some foods are instant hits, some they will never like, many others take time for them to develop an appetite on.
3) Chickens will go after the food that they need if they have a choice.
My chickens' main meals are 'all you can eat grains' (wheat, com, sorghum, Lupins, sunflower, Peas, Canola etc), I also put milk powder in their diet because grains are low on protein. On top of that, I give them meat/fish/prawns(mostly heads) every 2-3 days on average that they can eat to their hearts content.
There maybe other reasons.
My chickens get to stare at the vegetable patch all day long and only get to eat some when we give it to them ..
(picture taken a few years ago)