They don't. If a woman has a yeast infection and has unprotected sex her partner will get it to. He has to be treated as well or else they just pass it back and forth.
Do you mean just women, or that they seem to just appear?
If just women, it isn't true. Ask any guy who has had jock itch. That is a yeast infection. So is athlete's foot and thrush.
As for just appearing, not true either. Yeast needs moisture. So sweat and other body fluids can give the yeast the moisture it needs. Clean, dry clothes and some powder can prevent them. In the case of thrush, yeast infection in the throat, it is caused by not enough good bacteria. I have gotten it after taking antibiotics. So now I just buy lots of yogurt any time I am prescribed antibiotics.
They don't. Men have it too!
It's actually an equal opportunity infection!