Google Docs clones the Excel formula language for use in cells but allows you to write extensions/macros (where you would use VBA) in Google Apps Script but what is it really? It looks a lot like JavaScript.

It doesn't just "look a lot like JavaScript", it -- to quote Google -- "is a JavaScript cloud scripting language that provides easy ways to automate tasks across Google products and third party services and build web applications." [1]

[1] https://developers.google.com/apps-script/

Why not have a single language for the cells and for extensions?

Because programming in the extremely small is different than programming in the large.

With a suitable functional programming language, you could probably find something that would work tolerably well for both.