Earlier this year i was reading the bluetooth specification (of course not all of it, certain parts only, it's quite long) and I remember that there are pairing modes which can't guarantee that the connection is secure, because the method does not make sure that the connection cannot be eavesdropped by an attacker to obtain the keys that'll be used.
Of course better devices will already be paired in the factory.. but that's not all of them, and how do you even verify that it has been paired in the factory, or it just randomly pairs with whatever it finds in pairing mode? Or how do you verify that they correctly verify the incoming packets?
But for the user-pairable ones, the security of the connection depends a lot on what pairing mode will you use, there's a huge difference if you just press a button and done, or when you can somehow input codes on both devices.

And it's not even just about whether you trust your devices to follow the bluetooth specifications correctly. Bluetooth had many different security mechanisms over the years, for the different bluetooth versions, many of which don't protect against certain types of attacks and situations, or which are just plainly insecure.
But they still exist and they are still used by some (or more) manufacturers who just don't care.
Also keep in mind that for compatibility many Bluetooth devices also support communication with older versioned devices.

This stackoverflow post tries to summarize a part of the evolution of bluetooth security. Hopefully with it the above will make sense

But then bluetooth vulnerabilities are also not unkown, both software and hardware based.