Why is it that weight training is just for men?

It isn't.

But men are generally bigger, and can have a larger proportion of muscle mass, so they can get better at it.

Men in general are more interested than women in becoming stronger.

How much you can bench press is a more popular topic of discussion among men than among women.

Weight training can also make you more muscular, more stocky, which is a more popular look for men than for women.

But anybody can do weight training, and everybody can benefit from it.

If you're worried about what body shape you'll end up with, this is easily controlled through the type of exercises you do and how you do them.