The dominant factor is: there is a human who understands the entire system.

That is vastly easier to achieve by making a small, successful system, which gets buy in from both users and builders to the extent that the former pay sufficient money for the latter to be invested in understanding the entire system and then growing it and keeping up with the changes.

Occasionally a moon shot program can overcome all of that inertia, but the “90% of all projects fail” is definitely overrepresented in large projects. And the Precautionary Principle says you shouldn’t because the consequences are so high.