Why does viewModelScope.launch run on the main thread by default

ViewModelScope.launch { } runs on the main thread, but also gives you the option to run other dispatchers, so you can have UI & Background operations running synchronously.

For you example:

fun thisWillRunOnMainThread() {

    viewModelScope.launch { 

        //below code will run on UI thread.
        showLoadingOnUI()

        //using withContext() you can run a block of code on different dispatcher
        val result = novel.id = withContext(Dispatchers.IO) {
            withsomeRepository.someWork()
        }

        //The below code waits until the above block is executed and the result is set.
        liveData.value = result
        finishLoadingOnUI()
    }
}

For more reference, I would say there are some neat articles that will help you understand this concept.

Medium link that explains it really neat.