The biggest companies in this space maintain an active revolving door, which ensures that procurement policy is moulded (either consciously or unconsciously) to their process and needs over time. Even more insidiously, they've convinced governments to gut their own IT workforce, removing the people most qualified to critically analyze software vendors. This appeals to your average bureaucrat because it appears to strike a good balance between effort and risk minimization (e.g. why bother managing multiple smaller vendors or timelines?), while in practice it does exactly the opposite.