I think it has to with peoples inherent lack of discounting sunk costs. “We have already spent X amount. We must make sure it isn’t wasted, so let’s spend Y more and see if that’ll fix it”. At least they pulled the plug.
It often isn’t clear that spending Y more won’t fix it, or that spending Z on a new project will - by the time you’re anywhere near half a billion euros into a project you’ve already worked around an incredible amount of unforeseen issues, lost cultural knowledge about them once they’re fixed, and there’s every chance that those issues (or more) will show up again if you start from scratch.