This one of those local things that makes possible to distinguish different ethnicities. Humans propagate physical traits genetically, therefore if a group of people has similar origins they will have some facial features in common as well.

Do you live in a culture where most men smoke and women don’t? That could be a cause, from receding gums.
Alcohol and diet could affect it as well.

What region do you live in?

I can’t say I’ve been paying close attention. If I were forced to guess I would have said the opposite: men have stronger jawlines.

Not at all where I am

I think that’s an eastern European thing if I’m correct here.

Could be this, if you live in society with high alcoholism rate:

Fetal alcohol syndrome (FAS) often causes a distinct jawline characterized by micrognathia, or a small, underdeveloped lower jaw

Don’t know why it would show in males only, but could be you expect stronger jawlines on males and notice more their absence, while on females it is less expected and evident (also more makeup use to somewhat alter appearance)