Streams are live broadcasts, so the channel will only show up when the person is live broadcasting. Our glossary defines the terms. 
I think you’re looking for the Get Channel by ID endpoint.
User IDs were necessary for name changes. There are a lot of APIs that refer to users by ID. Facebook, Twitter, etc. We do have the endpoint to get the user object by login, and you can send up to 100 logins to the endpoint: https://api.twitch.tv/kraken/users?login=dallas