Storage data structures. Database tables are designed for fast read/write. Excel is designed for fast simultaneous parallel computation.

To get a sense of what this looks like, you can read more about their data structures; Databases typically store data in what's called a "B Tree" and spreadsheets typically store as a format that can be easily converted into a "Directed Acyclic Graph" (although Excel lets you turn off the "acyclic" part if you allow circular references).

Although, with Excel specifically, there's probably not much difference since it has some database functionality now.