ask Congress, it's their law. but one would assume to prevent it in cases where one spouse had say 210,000 in AGI and the other's AGI was 10,000 or under.
could it have made a provision in the tax laws for this situation - yes. but spouses who MFS don't have to disclose their info to the other spouse
there is an exception to this limitation
if you MFS and lived apart at all times during the year you are treated as single.