Why do you think that mens sex drive stays the same while women over a certain age lose their sex drive?

Drive may stay but. In men performance dwindles over time. Peak years for men are late teens and through the twenties and maybe early thirties. In women their peak years are about early to mid thirties. Women vary in their interest in sex more widely than men for reasons that include past sexual history, if they have children or not and how good relationships with men are. Men lose performance mostly, women interest.
Primitive drives. Most women over the age of 35, happily married, do not have the same sex drive as their husbands. Men initiate, women need to be coaxed. There is nothing wrong with either of you. Evolutionary drives cause men to need to sire more children and women to spend their time and energy caring for the children they have. This is a case where primitive hormones have not kept pace with modern society.