[Jorchime]

I think it is because of what is described through Conway's law: https://en.wikipedia.org/wiki/Conway%27s_law

    Any organization that designs a system (defined broadly) will produce a design whose structure is a copy of the organization's communication structure.[2][3]
    — Melvin E. Conway

The problem isn't necessarily software itself, but how we organize people (more than 2 or 3), how we communicate, how we mirror operations, expectations, etc. in software.

Scaling sustainable software feels like an "unsolved" Problem, because the society hasn't figured out how to organize better.

[ArjenM]

Small teams have honestly always been successful to the flaw that they outgrow themselves constantly in my experiences.

[Cthulhu_]

Small teams can be super productive, but unless the people in it are there for the full run of a product, you'll end up with a problem; taking over the work for one developer will take more than one developer.

I'm sure - but haven't witnessed this myself yet, so take it with a grain of salt - that if one productive developer builds an application in a year, it needs a team of 5-10 to continue development at a similar level, and even then it may not make it.

Companies need to focus on keeping software as simple as possible, well documented, and transferable. Unfortunately this also means curbing people's enthousiasm.