Why is there no attention aggregate?

I read this: 'Nama-rupa' is simply the aggregates: > And what, bhikkhus, is name-and-form? Feeling, perception, volition, > contact, attention: this is called name. The four great elements and > the form derived from the four great elements: this is called form. > Thus this name and this form are together called name-and-form. > > SN 12.2 Why is attention not one the aggregates?