Most of the nutritional value I've seen in cereals have been through supplemental ingredients. For example, a lot of the vitamins and minerals are added in as if you crushed supplements and put them in your bowl. There are better ways of getting those nutrients, including fiber: beans, chickpeas, lentils, peanuts, flax seeds, chia seeds, various vegetables, etc. None of those require milk to be palatable and they're much cheaper than the ultra-processed cereals. When it comes to fiber, you'll want a variety, cereals largely just rely on bran.