Earlier quoted context omitted.
"The rent of land, therefore, considered as the price paid for the use of the land, is naturally a monopoly price. It is not at all proportioned to what the landlord may have laid out upon the improvement of the land, or to what he can afford to take; but to what the farmer can afford to give." — Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations, Book I, Chapter XI "Of the Rent of Land"
This assumes that the farmer has to remain a farmer. This can be true for one generation, but the next generation can choose another profession. People don't become farmers if they have to pay too much.
How does it assume that?