It's not graft exactly, but simply the project structure leads to enormous inefficiencies. Eg subcontractors cannot work/meet/organize/plan directly with each other, because financial interests put silly lines in the sand. (Just think about how slow/dumb things can become if the backend, frontend and ops are completely separated, plus there's a middle layer (the general contractor) between them and the stakeholders.)

Plus, it seems that the main problem is that there's simply not enough experience/expertise at governments for managing/planning/executing these projects. Because too few people are in-house (or on payroll/retainer with the right/aligned incentives), and too few projects are done, so no competent team forms over time.