system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do public projects always seem to go overbudget? Often by up to 3 to 10 times? Eg IT systems, civil engineering projects?
gpt
To get funded there's intense pressure to quote the lowest possible figure for costs.  Politicians want a low cost estimate because it makes the sale of bonds or the raising of taxes more palatable to voters.  Most projects are awarded based on a bidding process and the lowest bid is usually the winning bid.  Once a project is underway it is commonplace for changes to be requested by the owner of the project (some layer of government) and when those changes are made the project managers figure out how to gross-up the costs of those changes to wring more profit out of the deal they usually underbid to win in the first place.  It is often well understood by everyone involved that the profit of the whole project will come from such change orders - and jobs are often put out to bid even when everyone knows that substantial changes will be required after the bidding is complete and during the project.

The "lowest bidder wins" contract award system has proven to be the worst possible option -- except all the others.  Not using a cost-based bid tends to unlock endless corruption since it's worth just about anything to influence the selection. And spending a couple hundred grand on bribes to win a billion dollar project is totally always worth it.