Hey Dylan, great question! It’s definitely worth thinking about when so very many things on our planet cope fine with just one cell. The reason we have more is all to do with how big and complex we are. When life got multicellular (things started being made of more than one cell), it started being possible for those cells to get really good at particular jobs – they didn’t have to do everything any more! In the end, we’ve wound up with tonnes of cells that are very good at particular, niche jobs (which lets us live complicated, successful lives), but because there aren’t really many that are excellent at everything we need the whole lot to work together.