Most lawyers come into the law having come from doing a degree in the Arts. It’s unusual for most of them to have come from a sciences or mathematical background.
Given the length of time it takes to become a lawyer and how much their hourly billing rate is and most good lawyers are super busy there isn’t much incentive for them to learn how to use Excel. Why bother - there is really only downside in terms of billable hours in using excel to educate their clients.
Also their support staff tend to come from administrative or legal clerk backgrounds. So they also tend not to excel savvy.
So given other lawyers and judges come from similar backgrounds you have a whole culture of people that are telling each other and their clients that the calculations required for law are too complex for them or clients to do themselves. Instead you need specialized pieces of software to do this.
Of course it’s in the interests of these specialized software vendors to perpetuate the emotions amongst lawyers that “these calculations are hard!”.