the air would be hard to breathe为什么不用被动

不定式做表语形容词的状语和句中主语构成逻辑上的动宾关系时,不定式应用主动形式,这样的句式限于以下形容词 hard,difficult,easy,comfortable,fit,unfit。
The book is difficult to understand。
He is hard to talk to。
These plants are difficult to look after。
The girl is easy to get along with。
The water isn’t fit to drink。