Some projects are too big for one person to reasonably complete on their own, especially if they have areas they're weak in. Or not working with someone else places limits on what they can make.

Like I can make video games, and I can make art for video games, but if I want the game to have a decent chance of becoming popular, I'm likely going to need to work with a proper artist at some point. The games in which I did look a lot more interesting and attracted the eye more than the games I've made all on my own, at least (and I've done both).

Also, I can make video games by myself, but I only have time to make small games if by myself (currently working on a small game now in my spare time). I could potentially tackle a larger and more ambitious game if I worked with another programmer.

Realistically I'm never going to be able to make a Stardew Valley on my own, at least not working on it full-time for years like that guy did, his girlfriend paying most of their expenses for several years.