If you made a peach smoothie out of nothing but pure peaches, it would be identical to eating the peaches, I'm sure.

I am also not a nutritionist, but from my reading processed foods are bad because (1) they just have far fewer nutrients, vitamins and minerals that our body needs to function, (2) the additives and chemicals that are added sometimes harm our body directly and/or require the body to dedicate resources towards removing, or (3) they harm our microbiome by feeding bad bacteria which similarly produce stuff that isn't good for your body and damaging the good bacteria which we need.