Comparing Java threads and goroutines doesn't really work because Java's thread model maps threads 1:1 to OS threads, whereas goroutines are simply continuations that can consume any OS thread. Java threads couple both the concerns of scheduling and task definition. Java Runnables address only the concern of task definition and are closer in nature to what goroutines are, but they still don't possess the ability to suspend and yield their execution.
You can implement Go's thread model (more generally known as green threads, coroutines, fibers, lightweight threads) on the JVM. This is precisely what Project Loom intends to do and what Scala effect systems already do. You construct a program in a type called IO that only describes a computation that can be suspended and resumed, unlike Java Runnables. These IO values are cooperatively scheduled to run on a fixed thread pool. Because these IO values just represent continuations, you can have millions of them cooperatively executing at the same time. And there is no context switch penalty for switching the continuation you are executing for another.
The 1:1 mapping is just an implementation detail of of current JVMs. Originally green threads (with an M:N scheduler) were used instead. IIRC they weren't even guaranteed to be preemptively scheduled, I don't know if this has changed since.
Technically os threads are also simply continuations.
You can implement Go's thread model (more generally known as green threads, coroutines, fibers, lightweight threads) on the JVM. This is precisely what Project Loom intends to do and what Scala effect systems already do. You construct a program in a type called IO that only describes a computation that can be suspended and resumed, unlike Java Runnables. These IO values are cooperatively scheduled to run on a fixed thread pool. Because these IO values just represent continuations, you can have millions of them cooperatively executing at the same time. And there is no context switch penalty for switching the continuation you are executing for another.