Why do men's bicycles have a top tube while women's bicycles don't?
Frames designed for women do have that part - its called a top tube.   

Historically women wore skirts, so a lower frame in the middle made it easier to mount and dismount, and was less likely to accidentally show an ankle.

This picture shows a modern "woman's frame" with the top tube paralleling the downtube, and attaching to the seat tube lower down.



Positives: 


Skirts
Ease of mounting


Negatives:


Bike is flexier because of fewer triangles
Bike is built heavier to reduce flex


Another frame designed for with these positives is the Mixte frame, where the top tube is replaced by two separate tubes similar to chainstays and seatstays.  These pass around the seattube and terminate at the headtube and the rear dropout, increasing the stiffness of the bike.    They're still heavier than a normal Diamond Frame bike.



Here's a photo of a Peugot showing the twin tubes.



Finally - there's absolutely nothing wrong with a woman riding a DF bike frame.  The main differences now are about proportions, not whether the rider wears a skirt.  There are situations where a man fits a "woman's frame" better than a "man's frame"