@Kelli94 Because that's how the platform is set up 🙂 There has been discussion here on the forums before as to how that ought to be eliminated, but as Airbnb is all about getting guests to book and collecting their booking fees, they want to encourage you to pre-approve.
But don't fret- you actually don't have to click on either of those options if the questions come in the form of an Inquiry (a Booking Request is different). Simply messaging the guest back within 24 hours is sufficient and won't affect your response or acceptance rate or anything else. You'll still get messages from Airbnb urging you to Pre-approve, but you can just ignore those.
If you feel okay about the guest, go ahead and pre-approve, but since declines are counted against you, never decline an Inquiry- just message the guest back, answering their questions, or letting them know why what they are asking for isn't possible and wishing them well finding a listing that better meets their needs.