One of the problems which arises with guests who are new to Airbnb is that they don't upload their payment information until they want to book something for the first time. Then there is a waiting period while Airbnb confirms your payment info, which can take up to 24 hours.
What new to the platform guests should do is enter all their info (payment info, write a little blurb about yourself on your profile, upload ID and a clear face photo) a few days before they go to book a place, that way Airbnb has time to verify your information ahead of your booking request.
Not only is it an inconvenience to guests to have to wait to see if their reservation is confirmed, it's also a hassle for hosts, as the request blocks the host's calendar until the payment is confirmed. Sometimes there is some issue with the payment, and it doesn't go through, so the host has lost out on potential bookings for those blocked dates that fell through.
What I have found is that when requests come in from guests who already have a history with Airbnb, as soon as I accept the request, the booking shows as confirmed. If the guest is new to the platform, there is a wait while the guest's info is verified, because the guest didn't bother to open an account and upload all their info until they actually needed to book something.
@Terrell13
