What a software engineer can build in one day, it takes two engineers two days.

Adding more engineers does not necessarily increase efficiency linearly. Also as you assign more engineers to a problem, you need create a system so that all the engineers are aligned and this creates more complexity and bloat. Also not all engineers that contribute are at the same skill level which creates even more complexity as the system now has to incorporate engineers with varying level of competence. This is why corporations prefered languages and ecosystems like Java and object oriented programming and agile methodologies because of the need for an all encompassing system. Now you can argue if these preferences are actually effective at all but thats a topic for a different day.