There is a tendency in many cases to count a future inheritance as a certainty, often, for example, in the case of an only child or only sibling of a childless person. This is particularly so if the heir has seen the will or otherwise been told that he or she will inherit.
However, until the beneficiary actually does inherit, the inheritance is purely speculative or contingent. Many things could prevent it from coming about: the heir could predecease; the testator could change the will (or create a will, if the beneficiary was relying on intestate succession); the money could be spent or stolen etc.
When assets are divided or distributed in a divorce, only actual assets, the money or property which a spouse actually owns at that time, will be considered. This may involve setting a rule for dividing or apportioning a future income stream, such as from a business or real estate, but that is a future income stream that one or both spouses actually currently own.
Inheritances are simply too speculative to be considered. Since an potential future inheritance does not yet belong to one of the spouses, but rather to the person who would leave it to that spouse, to “distribute” it would be to distribute someone else’s money, something the court cannot do.
That is not to say that a future inheritance could not factor into a settlement arrived at voluntarily between the two divorcing spouses. Nothing would stop one spouse from offering to accept less property or less support than he or she would otherwise be entitled to in exchange for receiving a distribution from a future inheritance; though since that would be so susceptible to manipulation and change, it would be unwise in the extreme. Similarly, lawyers, judges, etc. are all human, and an awareness of likely future inheritance could certainly color how they view the resources or future needs of a spouse.
That said, a “future inheritance” is like a future raise, future lottery winnings, future gifts—it’s not something that a person actually owns or has title to, it’s not guaranteed, and so is not something that can be distributed in a divorce.