The five year project likely represents a bunch of engineers that are stuck in meetings and coordination for 75% of their time, then you staff the project with more people (because you need four times as many) then they all need more meetings to coordinate too. And then they need to write a lot more documentation in the process because in that five years the people working on the project will all turn over at least once if not 2-3 times. Never mind critical people getting pulled off the project to work on other more important things on a monthly basis as is common at large firms.

It's entirely possible for a 2-3 month project if one senior person was left the hell alone to do it to take a dozen people five years.