You do not debate the destination
The first reason is simple: you do not debate the destination. Choosing together is often the longest and least lovely part of a trip. A surprise skips that negotiation. Someone already settled the place. Both of you get to be there.
That is not indifference. It is letting the person who built the gift make one large decision, and saving the conversation for what you actually live together: what time you head out, what you want to walk, where you sit down to eat.