Actually, they do profess to be experts at building accounting systems. Accenture was spun out of an accounting firm and as you mention they've been building accounting systems since 1953. Building payroll and other accounting systems is one of the main things they do.
So if they're doing it for 56 years then why don't they already have a modular
offering where they can just mix & match the modules as required, akin to SAP?
I know the answer(s) ofcourse, but couldn't resist asking...