Because this formula made a logical mistake of "four words". Poetry in the major premise is a concept in the collective sense, while poetry in the minor premise is a concept in the non-collective sense. This kind of error, which is three words on the surface but four words in essence, is called "four-word error".
To put it simply: Li Bai's poems and songs can't be read in one day. "Poetry" here refers to all Li Bai's poems.
Viewing Lushan Waterfall is a poem by Li Bai. The "poem" here refers to a poem by Li Bai.