Let's break down how this would work in practice at, say, GF.
Screening guests who are checking in would require a thorough hand inspection of each guest's luggage, since as others have said, guests legitimately carry all sorts of metal stuff when they travel.
The bigger problem, though, is that if you only screen guests at check-in, you're only catching terrorists who have reservations. So, we'd need to set up metal detectors at the following points of entry:
-parking lot
-bus depot
-monorail station
-marina
-dock for boats that go to Poly and MK
But wait - GF doesn't have a fence around it, so anyone could walk in from the surrounding area, at any point around the resort perimeter. Or, come in by boat from Seven Seas Lagoon. In order to make metal detectors effective, you'd have to put a fence around the entire resort, including the areas that face the lagoon.
Now multiply that cost and effort by 30 resorts. And you still wouldn't have completely prevented a terrorist attack, because you don't need metal to make a weapon.
Are the resorts terrorist targets? Of course. Every public gathering place is potentially a target. But when you're running a vacation resort, you have to strike a balance, because if your resorts have the ambiance of a supermax prison, guests will stop booking rooms there.