Why are my notes visible without my entering a password?
Like most of the content stored on your computer (email, documents, etc.), the Evernote database is not encrypted. When you log in to Evernote for Mac or Evernote for Windows Desktop for the first time, your login information is stored and used to authenticate your app with the Evernote Service for the purpose of syncing. A copy of your notes is downloaded to your computer and stored in a database folder so you can view and edit your notes, even without an internet connection.
If this is a concern, you may consider signing out of the app after each session, using a strong operating system password, using whole-disk encryption either via your computer's operating system or a third-party utility, or using Evernote Web exclusively.