Because normal contracting for things, when you're spending government money, you're spending our money, tax money, and the people who are in charge of that are very aware of it so the temptation is to control exactly how you spend it.
Normalement, quand on fait un contrat, quand on dépense de l'argent public, on dépense notre argent, celui des impôts. Les gestionnaires en sont conscients ce qui les pousse à vouloir contrôler exactement la destination des sommes.