Why is their not a way to prevent people from seeing and connecting to my Bluetooth

When you go to Settings/Bluetooth it will show all devices within range of your phone that are in Pairing mode. But if anyone with a BT device tries to connect you will see a dialog asking if you approve the connection.

No one can see your Bluetooth unless you go to Settings/Bluetooth; while you are in that setting your BT is “discoverable”, but it is not visible at any other time. And no one can ever connect to your phone unless you approve the connection, and again, only when you go to Settings/Bluetooth.