I think for some of us it's about consistency. For example, fresh fruits can range from mushy to squishy to firm and tart/sour to sweet. Processed foods are the same every time all the time, there aren't bones or bruises to work around, and the taste is much more predictable.

As an adult, the health benefits of fresh foods can help tip the scale in favor of extra prep time to lower the inconsistency for healthier meals and snacks, but when you're a kid you have to rely on a parent/adult to understand and do the prep work (and not harass you about food waste when you leave mushy blueberries on the side of your plate). Processed foods are already consistent, and don't involve prep time by parents who are probably stressed and overworked.