Another reason is the fact that these large projects rewrite business processes of the organisation (instead of merely automating them). This obviously opens up

Another reason is the fact that these large projects rewrite business processes of the organisation (instead of merely automating them). This obviously opens up a huge turf war on how the post-deployment world is going to look like (esp. if there are to be layoffs). Additionally, in .gov, the business processes are always at least partially defined by law, which in consequence needs to be changed. The actual software project is the easy part.