Why is "lady" used so often as a synonym for "woman", when "gentleman" often refers to specific behaviour?

I have noticed the way "lady" is used in many contexts as a synonym for "woman", but "gentleman" often only refers to specific behaviour of a man (although it of course can also be a synonym). It seems that the word "lady" has been degraded of its high-cl and polite ociations, when "gentleman" hasn't.