As the title says, why does Node not support several event loops in one Process? The idea, is that node spawns a user decided number of threads, which each have an event loop. When a new callback is added, it is scheduled to one of the threads loops. The reason, probably against this idea is because now each thread can cause mutation and other bad stuff, but you could make the user decide what threads share state and so have to be run on the same thread or have a message like event memory sharing concept. This would help with computationally heavy tasks, this would reduce the overhead of sharing data between node instances to achieve the same result and therefore achieve better performance for the hardware and more user requests.
To make it more clear, when I mean several event loops, then that would be like forking and creating several node instances to utilise multiple cores but instead node itself would spawn 4 threads all with one event loop, so there is a lower cost of communication because it is using threads instead if instances if you are doing lot's of data sharing, not workers that you spawn to do heavy work and have nothing to do with the event loop.