Why are 20th century contract law rules so often uncertain and discretionary rather than precise and rigid?

The basic reason is the perceived need for government intervention and,more importantly,intervention in a complex social environment in which one situation differs from another.How can anyone frame precise,technical rules to deal equitably with situations that cannot readily be foreseen and that almost certainly should be dealt with differently? Thus,we get uncertain rules that enable courts to decide according to the equities of the individual case.