Take fresh fruit for example. Oranges are more filling than orange juice. Apples are more filling than apple juice. Why?
It’s generally had much of the fiber taken out, which means too, that many of the nutrients gets lost too.
Perhaps though, the best example is rice. Whereas wholegrain rice contains fiber and nutrients, it’s been shown more than once (and specifically during World War Two in POW Camps), what a diet of just polished white rice does. One ends up with Beri-beri due to an absence of Vitamin B, which is removed along with the husk.
For the same reason, whole grain bread is more filling than white.
So you end up eating more of it, and getting less nutrient, less fiber, and just more calories in the form of extra carbs.