They largely are healthy diets and force one to eschew processed and nutritionally void foods.
But normal diets include fruit, veg, wholegrains, fish, dairy, water... they include all healthy foods as they don't exclude anything. Even the more unusual foods hyped up as super foods or whatever can be eaten without having to follow any special diet. And as I wrote above, when I was in hospital, for me healthy food meant low-fibre, high-calorie foods, so processed food was often a healthy choice. But mainly you can have a perfectly healthy diet that includes processed foods or junk food or whatever you want to call it, as long as it includes all the nutrients, calories, fibre, etc. that you need as well, and as long as the amount of calories, fat, salt, and everything else that is unhealthy in excess is
not eaten in excess.
A "normal" Western diet can be healthy: the hospitals I've been to always offer everything needed for a healthy diet. And SCD, paleo, etc. can be unhealthy, as it's surely possible to consume too many calories, or too few calories, or follow the diet in many other ways that make it unhealthy (plus the obvious fibre issue
). And in a hospital - well perhaps this isn't as relevant in other countries, but here in the UK where we have free healthcare - the food has to be cheap. Processed foods that are easily available must help in that respect: processed food is generally less likely to spoil, so can be bought in bulk with less potential for wastage, and might be cheaper or more easily available. And I'd imagine that increasing the number of meals requiring separate preparation is going to mean more man hours. So while hospitals have to cater for special diets needed for health, religious or ethical reasons, adding in more specialised diets that aren't necessary isn't going to be a good idea.