Because it's not just junk food...it's M&S junk food. :-)
Like all things, in moderation it will probably do no harm. But if you live on takeaway and processed food it will probably cause you problems. I have lovely neighbours but they (and their two young children) live on what I would describe as junk. They have a varied diet - McDonalds one day, takeaway curry the next, Chinese the next, takeaway pizza the next, takeaway kebab the next. All of this is delivered, mostly on the back of a scooter, so tepid at best by the time it arrives. Sometimes they will have two deliveries in a day (lunch and dinner). I've even noticed that Greggs deliver their wholesome sausage rolls (you know, the ones where customers were variously "outraged" and "devastated" when they went u by 5p recently). I really don't think it is doing any of them much good.