Because identifying with anything particular to the dream character only maintains the dream.
To see the whole dream as “I” is the key, because the dreamed grass and the dreamed mountains and the dreamed houses and even the other dreamed characters are all equally manifestations of the dreamer.
Next, to identify with the dreamer of all the dream phenomena, including sense perceptions, thoughts and feelings.
The dreamer is the featureless background “I” of the dream itself.
As the dream character becomes aware that “this is just a dream” awakening within a dream, completely lucid, she becomes utterly unconcerned about any contents within in the dream.
This is quite similar to awakening.