When you're a programmer, you understand that the processor is dumb and has to be guided through absolutely everything. If you don't factor in the corner cases, your program is going to screw up somehow.

Legal drafting is the same thing, just replace the processor with a judge and the programming language with English. If, while writing a contract, you precisely define everything and cover all the ins and outs of what can and cannot happen and what should and should not happen in the event that any or all of the above likely or unlikely events shall happen during the lifetime of the contract. Then if you ever need to get the contract enforced by the judge, you can clearly point out exactly how the opposing party already agreed to resolve the situation in some way.

The real reason why you need to hire lawyers has more to do with making sure that the contract you draft is enforceable (considering current law and legal precedent), or else risk having the entire contract thrown out later by the judge. AKA legal system faulting.