I can't remember the full and exact reason for it, but indeed, many cereals have very little nutritional value of their own.
Recently, that guy on BBC whose name escapes me, made 'home made' cornflakes, and believe me, the process they go through is not very appetising, and indeed, the extras were added to actually turn them back into something resembling food - nutritionally speaking.
It is not such a bad thing though on the whole, as so many people have not got the first clue about balanced diets, and where to get the right nutrition, especially for their growing kids, so lacing their favourite cereals with vitamins etc. is a good way to ensure they get something - even if not ideal.
Apparently however, the iron used is pointless - they literally use iron filings, which is all but indigestible.