It can be done, and has been in Rhino, Nashorn, and graal.js. The problem isn't in the language per se but in the libraries provided as standard (mutexes, thread safe hashes/property access etc.) and in the implementation. It's much easier to build a JIT when you don't have to worry about concurrent threads of execution, and converting a single threaded JIT to a robust multithreaded one can take a lot of work.
