Yes, they do eventually divorce but the proposals took place from the time she was married until the divorce. On their second (or first) anniversary Mr. Skeffington was even chuckling as he handed one of the suitors his hat as he was departing (after once again having his proposal turned down).
Maybe it was because she was just SO beautiful men couldn't control themselves.
But also, I think, it's a device to show how cynical everyone was about the perceived "real purpose" (= money) of her marriage. In other words, nobody seemed to take Job seriously, as a MAN - including Fanny.
Another reason might be that they simply enjoyed the titillation of the flirt, knowing full well that she would not - COULD not - marry them.