Why isn't there medicine directly to aim at increasing female libido?

There is. It's called cash and credit cards.

I'm pretty sure Viagra works on XX people without penises.

I've noticed that a stack of $20 and $100 bills works.   They seem to overcome my social shortcomings and my less than ideal physical appearance. Go figure, huh??

There certainly is treatment available for this and any woman who feels low libido is a problem can go talk to her doctor about it. 

It's sad, there's not much out there.

Testosterone is responsible for libido in both men and women, and to give women testosterone results in a hormone imbalance and undesirable physical effects.