Look at 3e, and compare the mechanics for illusion spells to spells from the other schools, such as conjuration or transmutation. While they are all arcane spells, the manner in which they work is more like apples and oranges, rather than just different types of apples. They are just that fundamentally different.
Now look at spells in 4e. I take an existing wizard spell, vary the damage it deals, change the defense it targets, and voila...a new spell of another school? It is more like painting an apple orange and trying to pass it off as an orange.
I mean, what's the point of even bothering to differentiate them if they are all just minor variations of one another?