While I agree that many lawyers are financially and even numerically illiterate and having a basic knowledge of accounting and financial concepts would be very useful, you oversimplify the intricacies of legal drafting.
Yes, lawyers for most documents start with a basic form, could be a purchase and sale agreement, a partnership/LLC agreement, a credit agreement, a lease, a will/trust or a divorce settlement to name a few. But these agreements are negotiated with specific economic/business terms and allocations of risk which need to be accurately and unambiguously written. Parts of agreements interrelate to each other and the draftsman must make sure they work cohesively with each other. One of the biggest drafting error I see with young lawyers is they focus on one provision and in doing so create an ambiguity with another that they forgot to address. Many documents are tax or accounting treatment driven and how you choose to word the provisions may have different effects. In the field of litigation, you are trying to persuade someone (usually a judge) that the facts and laws support the outcome you want. If there is a settlement agreement, you need to write the terms clearly so that you are less likely to be back in court litigating the terms of the settlement agreement itself. Writing skills are still critical even in this age of computerized forms unless you are talking about very simple/standard terms and conditions documents which are lightly or not negotiated.