Why is it that fruits and veggies can detox the body?

They don't really: Your body cleans itself. Since most people prefer meat and dairy and complex carbohydrates and greasy stuff to fruits and vegetables, folks are ready to believe that eating just the latter will do magic. You'll be doing yourself a huge favor if you develop a taste for fruits and vegetables, but they won't magically remove imaginary "poisons". Spend the money on gym shoes and then use them.