Why is it important to have more than one person creating a game?
Depending on the scope of your game it comes down to what makes up for the final product.
Engine programmers (in house development)
tech & VFX artists
producers & project managers
Let's not forget the voice actors
Eh legal team too?
And then you probably have branches within those disciplines as well.
Unless you master all these skills and are super efficient at it, you don't need anyone else. But that's hardly the case. And if you take it upon yourself to do everything it will just take more time and effort.
If it's a small indie game that looks super simple you're probably fine on your own.
A medium sized game can be done by a small team.
For big games you have to consider all the additional people such as managers, marketing etc as well.
If you're using a full fledged engine many things are simplified for you. However you're still bound to creating each individual part ( art, music, code?).