The law, especially in common law jurisdictions like the US, England and a lot of the Commonwealth, does not work just by knowing the statutory rules. There are rules that exist independently of any statute. For example in England (I don't know about the US), murder is not a statutory offence - you won't find any act of Parliament that creates such an offence. It is a creation of the common law.
You therefore need to know all the relevant judgments to get a read on the way things are likely to go in a particular case. These are not necessarily expressed in clear, declarative terms. Judgments meander around mixing up the facts of the particular case with the legal principles involved, surveys of relevant authorities and bits of whimsy (read, for example, Denning's judgment in Miller v Jackson). In England at least it often happens in the higher courts that each judge will write a judgment. They will not all agree - sometimes there will be dissenting judgments. Sometimes judgments will agree about the outcome but for different reasons. Hence it can be hard to understand the legal principles on which the decision was reached.
Now, whilst in some areas it's probably possible to make an expert system that helps in some cases, you'd be crazy to rely on such a thing if you've been charged with a serious offence or have some significant civil dispute. And it's not just a question of trying to figure out which way it's likely to go - the way it's argued in court can make a big difference. You need to muster all the different arguments, based on legal authority, that support your position.