Ok, commercial engineer here, with knowledge of accounting and managing.
The thing with manual calculators is that they're more comfortable, reliable and quicker when you're working with printed numbers, which as we can see he has to do every day; not to mention the mental security that doing things "the old way" brings. It's common practice to use the computer and its accounting programs like Excel, but that's for when you have the digital data with you and/or need a copy for saving your progress, so when you have printed papers, physical bills or old-fashioned checks to work with, then making your calculations by using the computer would be tremendously odd, specially if you only need the numbers and will write it manually later.
It's like asking why there are people who still use bikes when we've got motorbikes now, or using sail boats when we have yatchs.