The average tenure of a software engineer/developer is just 2 years.
People come and go and the next thing you know you're using 6 different database with services written in 5 different languages.
Instead if you kept the same 6 engineers around for decades you could probably scale to 100m with those same engineers.
It one of the reasons why it seems like Google can never release a product. They can't keep people around long enough to see it through.