Because more engineers makes for a worse product, particularly if you're doing something very innovative that requires a lot of iteration.
The amount of code you can write grows linearly with number of engineers, but communication costs grow quadratically. Therefore, the speed at which you can execute is inversely proportional to the size of the team, unless you can cut communication costs. Established projects can do this with architectural decisions that prevent one engineer from needing to know about or communicate with other engineers working on unrelated parts of the product. New projects cannot, because the architecture hasn't been established yet. That's why every innovative product grows out of a small team.
The amount of code you can write grows linearly with number of engineers, but communication costs grow quadratically. Therefore, the speed at which you can execute is inversely proportional to the size of the team, unless you can cut communication costs. Established projects can do this with architectural decisions that prevent one engineer from needing to know about or communicate with other engineers working on unrelated parts of the product. New projects cannot, because the architecture hasn't been established yet. That's why every innovative product grows out of a small team.