Because law is based on precedent. This applies to halacha as well as western legal systems. You could have a system based on what you think is “good” based on how you feel today, but how would anyone know what is legal? Indeed, China abolished its legal system in the mid-20th century, roughly for the reasons you suggested, and finally had to reestablish it. With a set of rules based on established practice, no one knows what to expect. Therefore a lawyer needs to learn those rules, and the rules that resulted in those rules coming into existence.
Suppose you had to decide if was permissable to cross the street. Due to laws based on past practices and customs, we know which side of the street the cars are coming from, what a red light means, who has to yield. Suppose everytime you got in a car, you had to decide which side of the road to drive on, or whether a “red” light meant stop or go. Suppose you had to determine if the right of way went to the person with highest social rank (as was the case until recently).
Suppose you had to determine the meaning of the prohibition of “milk and meat” anew everyday? No checking precedents or past decisions. Some days a “cheeseburger” made with chicken is kosher, some days it isn’t. Some days the prohibition can effectively be ignored by getting meat from a cow certified as a beef cow, precluding the chance the milk came from its mother. But no, we look back at years of case law.
In truth, medicals also consult the past. Did you really think they have to invent a new antibiotic everytime you get sick? Do you think they dream up a way to set a bone or deliver a baby without finding out what millenia of experience have determined.