Is it just a side-effect of using V8 engine for the heavy lifting, or is it some part of the ECMAScript specification which forbids multithreaded implementations of the language?
Cons: All your async code becomes full of data races.
I suppose adding multithreading to the Node ecosystem would be as hard (if not harder) than removing GIL from Python. At least Python has locking primitives. Node, as far as I know, has none.
Atomics are probably the closest. https://v8.dev/features/atomics
I feel like most performance intensive nodejs modules drop down to c++ where real concurrency already exists. (e.g. sharp - https://sharp.pixelplumbing.com/api-utility#concurrency)
In all of them - JavaScript is mostly used for asynchronous programs, which are easily parallelizable. There's no reason not to want multi-threading.