And it kinda is. We provide a parser¹ and a runtime², and you can download a copy of any notebook compiled to a JavaScript module: https://observablehq.com/@observablehq/downloading-and-embed...
That said, we’re explicitly designing for the reactivity of the notebook environment. Working on fundamentally reactive code in a traditional text editor is ... alright, I guess. You’re still stuck reloading the whole enchilada any time you make a change. It’s nowhere near as exciting as working on reactive code in an editor that understands and respects the reactivity, and can keep your program running and re-evaluating as you tweak it!
That said, if you're interested something less notebook-y, check out what Rich Harris is doing with Svelte 3. It's totally a not-so-distant cousin.