Usually because it's raw and that's unappealing. It needs something, or perhaps just age, to help.
If it's seasoned, steamed, or mixed with something like cheese for broccoli or ranch and raw it's usually fine. Downside is you now get calories from the cheese and ranch dip. I blame the fact they get forced to eat the most unappealing form of it, and it becomes what they associate with all of it. I used to dislike veggies, but once I got into my teens it became tolerable/fine, such as broccoli and cheese, steamed/cooked whatever, mashed potatos mixed with corn off the cob so that you can use it as a bonding agent. There's healthier forms I've grown accustom to, but it took time to get past the barrier.
I think it's because they're given flavorless unloved generic stuff and think all of it's bad. Haven't come around yet. Bet there was a vegetable you weren't thrilled with, so imagine associating that with all of em due to bad experiences.
why do all the vegetables in your life have to be steamed? what about stir fired? baked? deep fried? roasted? ---