Complex pre-existing business rules of a large, sui generis employer (governments, especially sovereign ones rather than localities, often are because the laws that apply to other employees don't apply to them, and the rules that do apply often aren't coherent across-the-board policies determined by a central org focussed on HR, but often are set in special case laws applied to individual subunits of the government by a legislature then operationalized by the HR staff in that subunit) trying to be put in a COTS system make it hard, but probably what makes it most out of control is that the contractor is optimized in maximizing value extracted not efficient delivery and that governments generally suck at overseeing IT contracts (sometimes this is corruption, but it's often just lacking the knowledge and skills to do it effectively at the direct contract management staff level, often complicated by mandated IT contracting frameworks that are based on the underlying assumption that software engineering is basically structurally the same as civil engineering.)