Because, with the exception of very, very few women who held titles in their own right, the wives and daughters of almost all British nobles were called ‘Lady’. The wife would be called Lady Spencer (example wife of Earl Spencer, while their daughter was called Lady Diana. An exception would be daughters of the lowest noble rank, Baronet, who would have been introduced as The Honorable, not Lady.
The wives of those appearing in Birk’s Landed Gentry were also called Ladies.
Gentlemen could be used to refer to all Landed Gentry.
A greeting at a mixed table of nobility and gentry would be ‘Milords, Ladies and Gentlemen.