Stupid lens question

So I have a stupid lens question I've always wondered about.

Why is it that when you hold a lens up to your naked eye and look through it, objects appear out of focus, even when your eye is exactly where the sensor would be. Specifically, I'm looking through a 300mm prime and while objects 1ft away appear sharp, the distant objects it's supposed to be focusing on are all blur. For the distant objects to become clear, i have to hold the lens over a foot away from my eye.Why will the lens focus distant images for a sensor but not for an eye the exact same distance away?