Why does self-generation as a deity (yidam) require annutarayogatantra empowerment (wang, abhiṣeka), while visualizing it as a separate image doesn't?
Lighting a lamp, you need another lamp. Because during self generation you are the Buddha, and in order to have 100% conviction that you are the Buddha, you must be told(initiated) by the Buddha himself. That is why it is said that seeing/recognising the guru as the Buddha himself is important in Tantra. Further the guru himself must have an unbroken lineage starting from the Buddha himself.