Why isn't adultery grounds for an annulment?

An annulment (Declaration of Nullity) states that the marriage was never sacramentally valid. If the marriage was valid on the wedding day, nothing that happened later in the marriage—not even adultery—nullified it.

That said, adultery may be evidence that your son’s wife did not enter into the marriage with the proper commitment required for a valid marriage to come into existence. In the wedding vows, her ex-husband said these or a similar vows:
"I, N., take you, N., to be my wife. I promise to be true to you in good times and in bad, in sickness and in health. I will love you and honor you all the days of my life."
If it can be determined that he entered the marriage without the intention of being faithful, then he essentially lied when stating his vows and a declaration of nullity COULD be given.

On the other hand, if the marriage is determined to be valid, the reality of the situation may be that she is married to a man who is a bad husband.. Separation and divorce can protect her, the children, and assets, but it cannot free them to remarry.