The reason for this is because the Sandbox is simulated. There are not actual entry/endpoints you can connect to. Here is a snippet from the API Sandbox article:
Using the API sandbox
You can explore the Wowza Streaming Cloud REST API using the API sandbox. The API sandbox allows you to test and develop streaming workflows in a safe, separate environment that doesn’t affect your Wowza Streaming Cloud production environment. The sandbox is fully functional and can do everything that the real API does, except that it creates simulated live streams and transcoders, not real ones. Because the sandbox doesn’t use real resources, you don’t accrue charges when you use it.
I hope this answers your question and clarifies the sandbox environment. It simply verifies you are sending and receiving accurate workflow commands.