@Juan63 As I'm sure you're aware, not all dogs exhibit the same behavior. Some are fearful or nervous, some are aggressive, some are instantly friendly towards everyone. Many hosts have dogs that have to be "introduced" to the guests. My dog is quite large and a ferocious barker at strangers. She has never bitten anyone, she just looks and sounds scary. It's actually the perfect kind of dog for me, as I live alone in a rural area- she keeps me safe, but I don't have to worry about her actually hurting anyone.
Once she is introduced to guests and realizes they belong here by my behavior around them, she no longer barks at them and, in fact, is quite the sucky baby, happy of any attention they want to give her.
But it wouldn't work at all (and for various other reasons regarding my place) for guests to just self-check-in. Not only would she bark her head off at the guest, even if I said not to worry, she doesn't bite, and they aren't a person who is normally afraid of dogs, it can still be intimidating to have an unfamiliar 70 pound dog barking at you, while trying to determine if you're at the right house, where the guest entrance is, and how to work the key lock or entry pad.
Not to mention, it wouldn't feel like a very warm welcome.