Because it leaves out two basic essentials:
1. The existence of the eternal Supreme God.
2. The existence of the individual eternal soul.
Other than that, it's pretty okay.
Not really, they just don't recognize it as a duality.
Behold the void, it is thine own intellect, blissful and shining.
Also left out (a subset of the Buddhist position that neither eternal individual souls nor God exists), is that desire for sensual pleasures or enjoyment is not the only kind of desire. In fact, the Vedas state that desire is inherent in the soul and just needs to be re-directed to its natural object - God. Desire for God is natural and produces the greatest happiness, desire for the world creates the greatest suffering.
The "I am God" philosophy of Shankaracharya is veiled Buddhism, but (mis)using the Vedas to support his position; whereas Gautama Buddha rejected the Vedas. But since both Buddha and Shankaracharya were predicted in the Puranas and had specific missions, they had their uses. Although the philosophy they both taught is erroneous in many ways.