The literal answer to the headline question is "You can, just open Developer Tools -> Console and code away." But it seems the question in the article is actually "Why can't I write node.js code inside my browser?", followed by what seems to me lots of confusion between coding and program execution environments (web-browser or MacOS).

You can code in JavaScript on most browsers without installing anything. Sure, the UI of browser Developer Tools can be somewhat intimidating for newcomers, but that could be fixed with some good tutorials. But it seems the author doesn't want to just write and run JavaScript, he wants to use node.js specifically. But the whole point of node.js is to take Chrome's JavaScript compiler/interpreter out of its native webbrowser environment and have it run in a Unix-like command line environment, such as Linux/MacOS. So complaining that you need to know some of the workings of a Unix-like command line environment to use node.js seems very misinformed.

As for accessibility of programming in general, the core developers are usually not that interested in that, but for most popular languages there are plenty of third party tutorials aimed at laymen. Most programming languages are specialized tools; they shouldn't try to be everything to everyone. We don't complain that university Calculus profs don't also write arithmetic books for elementary schools either.