Why do Buddhists argue that nirvana is nothing in addition to the skandhas?

Why do Buddhists argue that nirvana is nothing in addition to the skandhas? I found this, and I hope it suffices to demonstrate that's what the Buddha taught: > "What do you think : Do you regard the Tathagata as being in > form?... Elsewhere than form?... In feeling?... Elsewhere than > feeling?... In perception?... Elsewhere than perception?... In > fabrications?... Elsewhere than fabrications?... In consciousness?... > Elsewhere than consciousness?" > > "No, my friend." > > "What do you think: Do you regard the Tathagata as > form-feeling-perception-fabrications-consciousness?" > > "No, my friend." > > "Do you regard the Tathagata as that which is without form, without > feeling, without perception, without fabrications, without > consciousness?" > > "No, my friend." > > "And so, my friend Yamaka — when you can't pin down the Tathagata as a > truth or reality even in the present life — is it proper for you to > declare, 'As I understand the Teaching explained by the Blessed One, a > monk with no more effluents, on the break-up of the body, is > annihilated, perishes, & does not exist after death'?" The quote seems to say that the Tathagata is not without form feeling etc.. If nirvana were something in addition to the aggregates, then I'm thinking the Tathagata would be *without the aggregates*. On the grounds that nirvana is all that the Tathagata is.