Supernatural tradition!

But really, most low end motels aren’t going to offer different room styles to choose from. It’s often ‘take what you can get,’ and two queen-sized beds is the most common format found at motels. If you want a room with a single bed, you’ll find that most of them are going to be king sized, and you have to pay extra for that, something Dean’s not gonna do.

Plus, a part of him is always saving that second bed for the possibility that Sam will be able to use it one day. Picking rooms with a single bed feel like he’s giving up.